CA2155035C - Method and apparatus for oil well stimulation - Google Patents

Abstract

This invention describes a method of stimulating production from an oil well by removing solid wax deposits from a production zone. An electrical resistance heater comprised of a packed bed of spherical heating elements is lowered through the tubing on a wireline and placed adjacent to the perforations. Solvent is pumped through the heater to raise its temperature by 200°C and then into the formation to contact wax deposits. The solid wax deposits are liquified and together with the oil and the solvent form a single liquid phase. The wax is then removed from the formation by placing the well back on production. Because the invention completely avoids the use of either water or gas, the saturation of the water and gas phases in the formation is minimized, thereby maximizing the mobility of the liquid phase containing the wax and facilitating the removal of the liquified wax from the treatment area before it reprecipitates. The packed bed heater has a large surface area and a large heat transfer coefficient, so high power rates (150 kW) can be achieved within a compact volume (6m long x 5cm id) without solvent degradation.By heating the solvent to a high temperature, a minimum volume of solvent is required, thereby minimizing production downtime and solvent costs. The burnout and catastrophic failure problem usually associated with resistive heaters is avoided due to the multiplicity of current paths through the packed bed.

Description

21~5~3~
-, Title: A llethod and ~ ..AI.- for Oil ~ell Sti 1At;
FIEI D OF THE
This invention relates ~n~rAl ly to the field of 5 extraction of hydrorArhon~, such as oil, ga~ and cnn-l~n~At~, from ul.de. yLuul~d reservoirs. More particularly, this invention relate~ to the stimulation and ~nhr-- L of production or ecv. Ly of such hy~lrornrhnn~ from ~uch reservoirs.
0 Rar------ OF THE lh .
Much of our current energy needs are met through use of hydr~cArhon~, such as oil, natur~l gas, and cnn~n~ates, which are Leccv~:~d from naturally occl~rrin~
deposits or reservoirs. Typically, such II~ILU~ Are 15 in a liquid or gas phase in the reservoir. Liquid IIYdL~ I.V. ~ are often pLuduced by pumping them from the reservolr to storage tanks or a flow line connected to the wellhead. The pumping or "lifting" costs include capital costs, such as the pump, the prime mover (i.e., motor), 20 the rods and the tubing, and operating costs, such as labour, royalties, taxes, and electricity. Because some of these costs are fixed, a certain production rate is required to make such Lecv.~Ly e-~r rAlly fe~Rihle. If the revenue generated by selling the Lecuv~:~ed 25 hy~lrnrArhon~ is less than the lifting cost3 to 80 recover them, then the well may be t _ rily closed up or p~ n l ly ghut in. In some ca~es wells may be Ltop_.~ed when new terhnoloy-y becomes available, and in other cases the well may be eopelled if energy prices rise, once again 30 3raking production and ~ ;v~._Ly ~ irAl ly attractive.
Alternatively, a E ly shut-in well would be plugged with c:olluL-~L~ and ~h~n-lnn~d alto~e~h~r.

Typically, an oil well will be shut in or Ah~n~ir.nPd when only 20-50 percent of the total oil in the reservoir ifi LeC:G.._L~_d, because it becomes l7ne~ to rontlnllP to operate the- well. This UIlL~:Cu.~ ed oil has 5 been re~ o~n i -~d as a lost ~ ~ne ~ e in the p_st and thus there lave been many te~ hnlq~o~ p v~o~ to stl lAt~
production rates and cù \~r~ l ly ~n--reAce the ul~cimate L~CCI._L,2' of oil from reservoirs.
There are a n~mber of reasons why oil and gas 10 well productivity may decline over time. For example, productivity riPclinPs if 1) there is insllfflr1Pnt pressure differential between the well and the reservoir, 2) the flow between the reservoir and the well is ob.,L.u~ed, or 3 ) the mobility of the oil is restricted due to relative 15 r ~ ty effects. ConvpntlnnAl production practice, such as wAtPrfl-~o~ g~ gas re-in~ection and the like, is ~ffective for r-int~lnirg reservoir ~ to u. __ -the first problem. Many different r~ can result in to the flow of fluid hydrocarbon from the 20 reservoir to the wellbore. For example, there may be precipitation of mineral scales, such as calcite, anhydrite or the like, in the formation, the p~LLoration tunnels ( located at the bottom of the well ) or the wel 1bore. There may be mobile inorganic fines, such as 25 clay or sand, which are carried by the flow of the fluid being Le V._L~d into narrow pore throats thereby ~ kln~
them. There may be clay mlnPrAl~ which swell under the lnfll~Pn~e of L.~cuve~Ly and which t - f ~ result in flow path restrictions and a flow reduction. There may be an 30 alt~rat~ of the sat~ratll~n of a particular phase of the well. For example, in a low r- -hil lty reservoir with a very low w~ter content, damage can be caused if ~ater contacta the reservoir. The damage occurs as a reductio-~in the relative r~ -h~lity (i.e., mobility) of the oil 35 phase.

~ 5~35 It is believed that one of the ma~or flow o~_L~ucLions which results in rJr~cl inin~ productivity i~
the ~c__ l Ation in the reservoir at or ad~acent to the well of solid phase wax. This wax may be due to either an 5 ~:c_ l ~tion of mobile waxy- solids with b-~h~e ~
plugging or narrowing of the pore throats in the reservoir rock or precipitation of sOlid wax due to temperature, es~uLe~ or composition changes in the hydrr,~rh~mR beinq ~c~v~ed. Such changes might occur ~t any point between 10 the reservoir and the storagQ tanks on the surface.
lol,au~ , because the wax is associated with the oil phase, any r l Ation of sûlid phase wax in the well tends to selectively damage the mobility of the oil phase and thus reduce the production of oil from the well.
Nany methods have been developed and pl,~,~osed to stimulate the production of oil in wells to increas~
profitability and extend the ultimate ~Cr~V~LY. One common and relatively successful te~hniTl~ is referred to as hydraulic fL~c.Lu~:. In this technique, a high pressure 20 fluid is used to Ll _Lul~ the rock formation, thus creating a channel which penetrates into the reservoir.
The LL~_Lule is subsequently propped open using a ~rAnl~lAr material, such as sand. The fracture bypasses hydraulic restrictions to the inflow of oil into the well by 25 creating a new open channel and also by ~YroRin~ a large surface area of the reservoir rock to the channel, thereby greatly increasing productivity of the formation n~ the bottom of the well. However, this terhniqu~ is sub~ect to failure if the ~ pa.lL is not 30 s~lrc~ f~ll ly carried into the new L- _LuLes made in rock formation. Further, it can be difficult to control the LL _Lu- lng process and if the L. ~_Lule s~rr;~r~n~:~l ly is e~Lel ded ~eyond the oil zone into a gas or water zone, then the well may become une - ' r to oper~te .
35 Hydraulic LL.. _Lu~lng can t~ ~ ly improve the - 2l~a3~

productivity of wells which have a productivity decline due to an ac_ 1 Ation of solid wax. ~lowever, such t~rhn i ~ does not rentove the existing wax damage or change the basic wax damage chAn;P~; it merely bypasses 5 existing wax damage . Thu~, productivity of a LL _ LULe~d well will often decline at ~ high rate due to the ~. _ 1 Ation of uax damage in the LL~ACLUL~: channel .
S~ t refracturing of the reservoir may provide an in productivity, but again productivity will 10 decline over time. s~ refracturing thereafter typically does not provide sufficient productivity 1nrr~PQ~ to be r-~- 'r. Such LLa_LuLlng may thus provide a short-term method of increasing production from a well, but because it does not address the wax 15 ~ 1 Ation problem, the problem usually re-asserts itself, resulting eventually in a 1088 of effectiveness f or the f rncturing method .
other treatments to stimulate wells include peLL~/L~ing the casing of the well with shaped charges to 20 provide rh~nn~l~ or peLLoLItion tunnels through which the f luids can f low . Again this t~rhn ~ qu~ provides a short term i ~.~. which m~y byp~ss, but does not remove, ac.;l 1 Ations of wax, nor, prevent the further Al lAtion of wax.
l~atrix acidization, in which an acid is pu~tped into a reservoir to dissolve forr-tinn rock and precipit~ted scnles c~m also 8~ l 1 At~ production in uells. However, for wells having solid wax damage, matrix acidization may not work effectively, as solid wax is ~nRoll-hle in acid. Because acidization ig lnh~rf~ntly prone to create rh~nn~l Fl along the path of " least re~istance", the acid often bypaB8e8 the low r~ h;l~ty wax damaged oil zone and instead 3?eneLLaLes directly into ~ water zone at the bottom of the reservoir. Thus w~x deposits can limit the success of acidization stimulation, 3 ~

even preventing effective removal of any dissolvable rock or precipitation which are wnx coated.
Another t~rhniql~ for sti l~tin~ production is thermal st; lAt~D. In the case of thermal sti lAtinn~
5 oil, water or steam heated above grade may be pumped to the bottom of the well to try to stimul~te production from the Leco.~L~ area. However, it has been found very ~iiffi~..lt to transfer the heat by steam, wnter or oil to the bottom of the well by reason of the thermal losses 10 which take place as the hot medium is being I ,~ poLLed down the well bore. (Society of Petroleum ~n~in~rs~ Paper No. CIN/SPE 90-57 OPTIMIZING HO~ OILING/WATERING JOBS TO
MTNTMT~R FOR15ATION DAMAGE by John N~nni~r and Gina 1 igl~r of N iger Rn~in~rin~ Inc. ) For example, in the ~hot oiling te~hniT~
crude oil, solvent or water is heated above grade to a typical t~ _~ LuLe of 100-125C and then pumped into the well. Usunlly the heated fluid is pumped into the annulus between the tubing and the casing . D~nA i n~ on the 20 particular situation, some fluid will ~c.. 1 Ate in the annulus, some fluid will flow into the reservoir, and some f luid will f low back up the tubing and out of the well .
Heat from the "hot oil" is lost through the casing to the rock sll~ro~nA;n~ the well. Heat is also lost in counter-25 current heat ~YrhAn~ with the f luid which circulatesupwards out of the tubing. I. ~ at the bottom of the well show that the bottom hole t~ _ LUL~
drop~ during the treatment and excessive volumes of hot fluid do not si~nifirAntly raise the bottom hole 30 t _ aLuL~. Typically, the heated fluid will lose its e_cess t~ - in th~ top 300-400 m section of the well dl~e to heat losses to the casing and the counter-current heat ~Y~han~e d~13crihed above. Due to the geothermal ~r~Ail~nt~ by the time the Nhot fluid" reaches 35 the production zone at bottom of the well, it is likely - ~15~3~

cooler than the casing and thus actually absorbs heat from the casing and the rock D~.Luu..ding the well. Thus for ~ost Arrlirati~.nR (for wells deeper than 300 m), the "hot fluid ~ arrive~ at the bottom of the well at A t~ ~
5 below the reservoir lLure. Because the bottom hole t~ LuLe decreases during treatment, waxy solids are likely to precipitate from the crude oil and be filtered out in the pores of the reservoir in the ~GUIJ . ..~.y zone as the fluid flows into the ~GCU~_Ly zone. Thus, although 10 the "hot oil" terhniq~lo removes the wax deposits near the wellhe~d, it often cauges an Al 1 Ation of the waxy ~olids in the peLroL~Lion tunnels and reservoir ~uLLuunding the well. Thus, the Ar~l irAtir~n of heat to the well by pumping "hot oil ~ into the well through the 15 annulus is inadequate to remove w~xy deposits in the formation and in fact usually leads to even greater formation damage. The hot watering technique ~Yr~r~c~nr~
- -rAhle heat losses and causes additional fr,rr^t i r~n damage (e.g., by increasing the water saturation around 20 the well, precipitation of inorganic scales, etc.), 80 hot watering is not an effective terhniqu~ for removing f ormation damage due to wax .
Another method of thermal stimulation i8 rl~R~d in CAnAriiAn Patent 1,182,392, dated February 12, 25 1985 in the name of Richardson et al. (see also U.S.
patent 4,219,083) which ~i~c1 r~8~8 a nitrogen g~s 7~n~r~A~tirn system to produce a heat spike in a water-based brine solution. In thL~ method, the salt water solution ~ GLyùcs a ~' irAl re~ction to release heat, t~gQtl--30 with nitrogen gas, aR it is being delivered down the well,thereby avoiding some of the heat losses a_sociated with l-A-~ ,LLing a hot fluid down the well as r~ ed above for the "hot oilll techniqu~; the salt water ~olution only become_ hot when it iD some way down the well. The _alt 35 water Rolution may then be shut in for a period of about 24 hours to ~llow the heat carried by the solution to melt 5~35 waY located in the ~c~, . LL~ zone . The ~ lo~;ure not~s that wax solvents may be flushed down the well prior to or after the in~ection of the heat-rroAI~ in~ salt wAter ~I-lutinn.
However, there are several inherent di~ndvantages to the method disclosed in patent 1,182,392.
Firstly, the wax is not soluble in the salt w~ter ~>]llt~- n~ 80 even if the heat developed is sufficient to ~elt the solid wax deposits, two separate liquid phases will occur (i.e. ~ liquid ~ L~C~L~ phase ~no]~A~n~
liquid wax and crude oil and a liquid aqueous phase including - ~on water and salt water solution). If the water saturation is high in order to get a 4i~ni~Ant rise then the relative r- -hil ity of the liquid hydrocarbon phase will be very low as: _ ' to the water and the mobility of the 11~ ILucaLL-JJ~ phase Cl~ntA 1n1n~ the wax will be o~,, LLU~; Led. Thus, the water-based fluid cannot effectively carry the melted wax out of the reservoir. E:ven if solvent is present in the formation, either by mean~ of a pre-treatment flush, or ~
post-treatment flush, the salt water solution and nitrogen gas pL~duced by the reaction will together greatly impede the solvent from coming into contact with any such melted wax, greatly reducing the treatment 8 effectiveness.
Past studies have shown the effect of water 8At.... r~t~.n on relative r ~h~ 1 ity (B.C. Craft and M.F.
Hawkins Applied Reservoir ~n~ina~rin~ Prentice-Hall, 195~) . The relative r --hi 1 ~ty curves (i.e. dat~) for a particular reservoir allow the flow rate of oil or water through rock pores to be calculated as a function of fluid sAt~rr.t~ nd ~La8~uLe: drop. For example, on page 357 Figure ?.1 shows that if the water saturation exceeds 0.85, then the ~ ~nin~ 0.15 volume fr~ctlon of oil will not be mobile. Fig. 7.2 of this ~feL..~ce also shows that 35 an increase in the water s_turation of ~ust 0.35 ~lo<-raA~9~3 2~ 35 the relative E -hi 1 ity (or mobility) of the oil phase by 100 fold. ~hus, if salt water solution is scrl~ez~d into the fnrr-t~nn~ the saturation of the water is ~n~r~ARed and the relative E -hil~ty of the oil/melted 5 wax phase will be greatly reduced. If the water s~Lu~Led f~r~-t~r~n i~ ~-.1.R~ ly contacted with a solvent, the solvent will tend to channel due to the r~lAt~nnQhi~
between relative E -hility and fluid sat~rat~-~n rl~Qrr~ hed above . Thus, the solvent cannot ef fectively 10 contact or ' i 1; ~ the melted wax. Thus, contacting the formatlon with an aclueous based heating fluid to be followed by a solvent is unlikely to effectively remove the wax from the pores of the reservoir rock.
FUrt~ e ~ water can be rl gi n~ to some reservoirs as 15 it can cause clay swelling or fines ~ tion.
What is desired th~LeLo~e is a method for removing the Ar l~tionR of solid wax from the fluid pARsA. _yD which _ Re the well to remove ~ ~ q to the flow of licluid hydrocArhonR being pLuduced from the 20 reservoir to enable increased licluid hydrocarbon production rates. Preferably, such a method would be in~ nRive to use and would be capable of being used without a great deal of inconvenience or alteration to the well itself. Preferably, the treatment would physically 25 remove any solid wax, and would be effective every time it w~s used. The method also would preferably not introduce any water - based licluids into the formation to ~void reducing relative ~ --h;l~ty~ and hence mobility of the licluid l~dL~JC~ . Such method would also avoid he~t 30 losses associated with LL~I..s~uLLing ~ fluid from a cold ~ocat~nn (i.e., the wellhead) to a warmer zone (i.e., the - h~le production zone), which could lead to a decrease in the bcLL -1~. t~ ~ and cause wax precipitation and L lAt~on~ resulting in formation damage.
35 swlKaRy OF 'rHE ~
According to one aspect of the present g invention, there i8 provided a well treating process to remove solid wax from fluid p~a~ ays betw~en the well and a DuLLou..ding u-~delyLo~l~d reservoir, said process in~:
selecting a solvent which is g~n~r~lly miscible with melted wax, pùmping said solvent down the well at ambient t' _ ~r heating said solvent below grade in the well at a posit~nn ad~acent to the wax to ~e treated to minimi~
heat losses from said solvent during ~ fi~ L Lation of said solvent to the wax to be treated, contacting said heated solvent with the solid wax to be removed to ~ ~ 1 i 7~ said wax without reducing ~:he relative E- -h11ity of the wax/solvent phase, and removing said solvent and said ~ i 1 i 7~el wax from said fluid pA~sa~ yD .
According to another aspect of the present ~ nvention there is disclosed a method of stimulating an oil well by removing solid wax deposits from a LLe~i area, said method ~ n~S
placing an electrical heater ad~acent the area to be treated, supplying power to said heater to cause a relea~e of heat while simult~n~ol~ly passing a solvent past the electrical heater to directly heat said solvent to a t- _ _Lu-e above the naturally oCc~rrin~ tre~tment area t~ - tULe~, but below the t- ~ at which unacceptable solvent degradation occurs, pasDsing the heated Dolvent into the treatment area to co~tact the heated solvent with the solid wax deposits to be treated to 11i7'~ the wax and to form a liquid phase ~ ~in~
oil, wax and solvent and then removing said liquid phase ~ont~nin~ said 'ili7gd wax from the treatment area, without lowering th~ mobility (i.em, relative 35 r -hil~ty) of the oil/wax/solvent phase within the 'Te,' ': area.

21~5~3~

According to another aspect of the present invention there i8 ~liQ~ln8ed an electrical heater for heating fluidg, c, Qin~
~ means for attArhin~ the he~ter to a source of 5 electrical power; and a resistive electric heating element means, said heating element means having a hydraulic p ~3UL~ drop there across of 20 mPa or less for a flowrate of 1 m~/day;
a heat transfer area greater than 10m2 per lm3 of 10 heater; and an electrical resistance greater th~m or equal to 1 ohm and less than or equal to 200 ohms.
31RIEF UIS~;WC1~.L_ OF ~
Reference will hereinafter be made by way of 15 example only to the attached f igures which illustrate a p ef~L ad ~ of the pre~ent invention and in which:
Fig . 1 is a graph depicting the relat iOnQh i r between solvent volume requirement to dissolve a downhole 20 wax deposit ( in m~ ~olvent/kg of wax) against treatment t~ , Lu~e in degrees Celsius;
Fig. 2 is a preferred ~ i of the invention;
Fig. 3 is a close up view of a _, of the 25preferred: ' ~ of Figure 2;
Fig. 4 is a cross-sectional view along line 5-5 of Fig. 3;
Fig. 5 is schematic of a part of a pL~cLL~d circuit;
Fig. 6 is a ~ Ailed view of a , - ' of Fig.
3;
Fig. 7 is a cross-sectional view through the ~- , of Fig. 6; and Fig. 8 is a circuit diagram of the preferred 3~ power circuit.

2155~3~

nRTATr.12n ~ _ OF THE o A
Up untll the present, the composition and 801-lh~ 1 ~ ty of wax has not been well understood.
~rypically~ wax has been treated as a single _ _ ~L, ' and 5 its sol-lh~ 1 i ty has been assumed to be a weak function of temperature . However, the normal pA rA f f ~ n ~ ( N_PA rA f ~1 n~ ) which precipit~te to form wax rl~posit~ in U~Lde~y~usld hy~lroc~rhnT~ reservoirs include species from C20 H"2 to Cqo H182 and higher. As -- ' ;1 -i earlier, the wax deposits are 10 associated with the oil or c ~ te in the reservoir and typically contain between 30 and 90 percent of the associated liquid hydrocarhon. When a wax d~posit precipitates from an oil or con~ nQate~ the composition of a particular wax deposit appears to depend both on th~
15 amount of each of the N-paraffins dissolved in the liquid hydrocarbon and the solllhil1ty of each of the N-paraffins in such liquid hydrocarbon . The ~ol--hi 1 i ty of a particular N-paraffin in a particular crude or conA~n~r,te i8 related to the carbon number of the paraf f in and the ~0 t~ Lu~ ~: and the 801 llhl 1 i ty r~ Le. of the liquid 1.~1. uc~rLu~l. Thus, as the oil t , t: changes, the composition of the wax depofiits changes. The solid wax which precipitates nnd A- l Ates downhole at high temperature tends to include higher l-~c~lAr weight 25 rArAffin~ and have higher melting points. ~see OPTINIZING
HOT OILING/w~rRl~ JOBS TO rl l ~ l M 1'~1' F~RM~ DANAGE by John ~ r and Gina Nenniger of 1~ r Rn~i n~ r~ r~
Inc. ) ~ L, because these wax deposits occur n~ ral ly at elevated t , _Lu ~B in crude oils and 30 con~i~n~rlt~ it i8 obvious that thes~ deposits contain highly ~ n~ h1 e p~r~ffins.
One of the t~hn i ~i ~ which has been used by industry to treat wells to remove wax deposits is to employ solvents; a solvent is pumped or "s~J~?z~n into 35 the f~r~tior~ to dissolve the wax. When the well is put - ~155~3~

back into production the solvent carrying the dissolved wax is then pumped out of the well. Although thi~
~erhniqup has been fLe~ Lly used, the composition of the wax deposit has g~nf~r~l ly not been known, and 80 the 5 gr,lllhility of the reservoir wax in the solvent is not known either . Fig . 1 shows a 801llh~ 1 ~ ty curve of the volume of a typical solvent required to dissolve 1 ki lr of a typical wax deposit as a function of ~. For a reservoir t~, of 40 C, more 10 than 2 m~ of solvent are required to dissolve ~ust 1 ki loqrAm of wax. In general, excessive volumes of solvent are required to remove wax damage at reservoir t LuLæ.
However, Fig. 1 also shows that if the solvent 15 can be heated to 70 C, then only two litres of solvent are required per kg of wax deposit. Although dif ferent solvents are slightly more or less effective, the effect of temperature ( i . e . the slope of the curve in Fig . 1~ is similar for many different solvents. Thus, one surprising 20 result $8 that the application t~ ,~~ LuLæ of the solvent is 80 critical in det~rm1nin~ the effectiveness and llqefl-l nPgs of any such solvent treatment . However, what remains is how to effectively heat the solvent to achieve the desired effective and useful result, namely, the 25 r~ 7~tion and removal of a gi~nif~ nt amount of the Al 1 ~t.~d wax deposits. In this context it will be appreciated that si~nifirS~nt means 8llff~ n~ removal of wax to --P-lr~hly increase production rates or flow rates through the treated area. In this context, to heat the 30 solvent, means that the solvent has had its , _ ~.Lu. el raised above the naturally o~C--rri r ~ L _ ~ of the reservoir.
Arcrrtl~ nq to the present invention there is disclosed an ~ L~IL48 ~nd a method in which a solvent is 35 beated directly ad~acent to the LLe~ area. Several 21~5035 different sources of energy could be used to raise the t~ of the solvent at the bottom of the well (e.g., exothermic rh~mirAl reaction, electrical heating, radioactive decay). However, electrical heating i8 S rr~f~rAhl ~ due to safety, control, r~1 i Ahi 1 i ty and cost considerations. The use of electrical energy avolds certain ~rnhl i nhc-r~nt in the heating the solvent via rh~mir~ll reaction. Firstly, it avoids the I .,.--~vLLation of bazardous rh~mirAl~ such as oYi~7;~rs and fuels.
10 Secondly, it avoids the ~7iff~rulties associated with initiating ignition and controlling the rhc-mirAl reaction, such a~ the rate of the rhPmirAl reaction and the hazards AR80ciAted with any 1- lete reactions, such as resldual explosive mixtures of gas or cnrro8inn. Electrical 15 heating also avoids formation damage due to the nYi~tinl7 of any aqueous species present. An example of this problem would be the oxidation of Fe~ to Fe~ and a 8llh~e~lu l- precipitation of Fe(OH)3. Lastly, any p_rtial oxidation of ~ L~C~ t-OI~R in a rh~mirA7 reaction heating 20 system can produce gums, tars or asphaltene-like material which could plug the pores of the formation and create even worse formation damage than the 801 i~ifi~d wax.
The generation of heat by dissipation of electricnl power can occur by several means. For example, 25 ~ductive, resistive, dielectric and microwave technologies can be used to generate heat from electrical power. Of these, a resistive heater r7~crrihed herein is preferred due to its compact size, Ql _lirity~ r~liAhility end ease of control.
Fig. 2 shows a schematic diagram of a preferred ~i of the invention. The f~T~i ~ shown consists of a number of ~ _ Q. A truck 2 is shown resting on a surface grade 4. An oil well i8 ~hown schematically and oversized generally as 6 with an outer casing 8 forming an 35 ~nnulus 10 around a tubing string 12. The tobing string - 215S~3~

12 E~,~__L~tes through a formation 14 to a ~_U._Ly zone 15 .
At the bottom of the tubing string 12 i8 an opening 16 which allows fluid i-~at~nn between the 5 tubing string 12 and the annulus 10. ~ ~ pelLoLe~LionR
18 are provided in the outer casing 8 at the Lecu~,_Ly zone 15. $he peLrn~ti~n~ 18 allow fluid c ~ration between ~he annulus 10 and the ~O~LY zone of the forr-ti~ 15.
Also shown above grade are an electrical 0 9~ sr ~n~l~rat~rl 8~ irAl ly at box 20 which has power outlet cord - ~irg electrical Cùlldu~:~OI 22. The g~n~r~Ator 20 i8 preferably of a portable diesel electric type, although in situations where the well 6 has an adequnte supply of electrical power, the y~ LntOI 20 may 15 be replaced by a conventional electrical power grid hook-up, along with ~,upLiate transformers, rectiflers nnd controllers. ~pf~ t on the application, it may be advantAgeou~ to convert the alternating current (AC) power to direct current (DC) as more power can be carrled by a 20 given conductor 22 in DC operation and inductive COurl i n~
between the cor,du~ Lol 22 and the tubing 12 is also avoided .
The next _ L iB a wire line assembly, which ~ nr l ~ a winch 2 6 which raises and lowers the 25 conductor 22 within the tubing 12. The winch 26 is operated by a gas or electric motor or the like. The insulated COI~UULUL 22 passes around the winch 26 and through a lllhr~Ator 28. The lubricator 28 facilitntes the passage of the insulated cond~.Lol 22 into and out of the 30 ~r~ l 1 hP~d of the tubing 12 . The lubricator 28 is also adapted to provide a ~ DULe seal around the cables a~
required. The winch 26, l-lhr~ ator 28 nnd electrical g~n~rAtor 20 will be f~ Ar to those skilled in the art.
C~ e~l .Fu~l ~y they are not described in any further datail _ 15 --` ~
herein .
The electrical l,UI~dU~:LOLD 22 are preferablr in the fûrm of inQul;-ted electrical cables. Where the depth of the well is such that the strength of insulated cable 5 is irA~ Jtte, such cables could bQ r~plA~ l or ~L~ ed onto the sucker rods ~not 5hown) whieh are usually used in the ~ell to raise and lower the pump. If the sucker rods were used as a co~.lu ;LDL, they would have to be electrically i QO1 ~t~ to prevent contact with the 10 production tubing. The electrical power would then be transmitted downhole through the sucker rods. A further alternative would be to use the tubing 12 itself as a part of the electrical circuit as jC~Q.-r;h~ in more detail below. However, this alternative would also recluire 15 a~ccJpLlate electrical isolation.
At the bottom end of co~.lu~LoI 22 is shown a set of ~ars 27 and a resistive heater 30 which are shown in more detail in Pigure 3. The ~lrs 27 are slidably connected to the conductor 22 and can be used to supply a 20 sudden impulse (~erk) to the heater 30 and thus free the same in the event it becomes _tuck downhole. A contactor 32 is also shown which is lt; 1; - i when the tubing 12 is used as a .;UJ~duu LOI to return the eurrent back to the wellhead and to the ~J.~ l c~i 20 thereby completing the 25 electrical circuit. Thus, the contactor 32 may be required to provide a good electrical contact between the tubing 12 and the heate~r 30. Alternatively, the où~ .cLoc 22 could allow the current to return to the 9~ ~.c,t~,r 20 via a return insulated electric~l power line.
The internal ~LLU~;LU~e: of the resistive heater 30 is ~hown schematically in Figures 3 and 4. The heater 30 is attached to the ~ars 27 by a ~o -rl; r~ 42 . The heater 30 has a slightly enlarged CiL. r ..ice 44 to seal against the pump seating nipple at the bottom of the - 215~035 tubing (shown in Figure 2 as 29) to prevent solvent from bypa~sing around the outside of the heater 30. The heater 30 has fluid p~Q8P3~ _yD or holes 43 in a threaded endcap 46 at ~he top to allow solvent to flow into the heater 5 body 30. The solvent then flows through holes 47 in an upper distributor 48, through a packed bed 50 in a manner as h~r~ After ll~rrihed, through holes 51 in a lower distributor 52 and out of holes 53 in a threaded endcap 54 at the bottom of the heater 30.
Figure 4 shows the heater 30 in cross-section through line 5-5 of Fig. 3. A "+o channel member 56 B.,rAr~tP.R the packed bed 50 into 4 channel 8-, lAh~lled A, B, C and D. Also shown are inner liners 58, which may be , ~sed by set screws 60 threaded through 15 an outer heater shell 62. The set screws 60 may be used to compress the packed bed 50. Such ~ sion facilitates electrical contact between ad~acent packing ~1 ~ as ~c~rihed in more detail below. The set &crews 60 are located at regular intervals along the 20 length of the heater.
The electrical circuit through the packed bed 50 is shown schematically in Figure 5. To prevent ~lectrical short circuits the packed bed 50 and distributors 48 and 52 are electrically isolated from the 25 ~+~ channel 56 and the inner liner 58 by an ~n~ Atin~
coating r^t~rlAl 64, such as a rubber, plastic or plasma sprayed ceramic. The upper distr~hut~r of channel segment A is connected to the power input from the CUnlU~ ~OL 22.
The current then f lows to the bottom of channel A of the 30 packed bed 50 and then through a co~ LoL to the bottom of channel B. The electrical current then flow~ up channei B to tha dLstributor at the top of channel B. The current then flows through a ~o~cLor to the top of channel C.
~he electrical current then flows down channel C to the 35 distributor at the bottom of channel C, through a 2l~sa3s or to the bottom of channel D, up channel D to the distrlhut~r at the top of channel D. This distr~hu~or is in electrical contact to the header body 62 through a c~ e_Lo and the current is eLuL~.~d to the wellhead ~,nd 5 the g~ LaLO~ 20 through the tubing 12 or else a second ~on~u~LoL 22 to complete the electrical circuit.
The lower distr~h~ltor 52 i8 shown in more detail in figures 6 and 7. Figure 6 is a plan view of the lower distrihllt~r 52 showing a contact plate 80 which acts as an 10 ~l~rtrir~l connector between channel fi-_ ' 9L D and C. The contact plate 82 acts as an electrical connector between channel se _ ,~ A and B. The contact plate 80 is; ~nl Ated from the contact plate 82 by an insulating material 83. As shown in Figure 7 the contact plate 80 is DU~O' Led on the 15 insulatir~g material 83, which, in turn, iB supported on a backing plate 84.
. .
It will now be appreciated how the preferred electrical circuit of the present invention is configured.
The electrical power is supplied by a variable vol~age 2 0 d rect current ( DC ) power supply . DC power has several aevantages over alt~rnA~in~ current (AC), as ionc-d before. The electric power is ~urrl ~l~d by a direct current variable voltaqe 200 kW portable diesel electric power gen~rAtor. The volt~Lge i~ controlled either manually or 25 aut --ir;llly on the basis of a ' Lu ~ meaDu ~ : in the heater, and the maximum current is limited to 150 amps to avoid ov~rh~atin~ conductor(s) 22. Figure 8 shows tb,e electrical circuit ~c~ ~r~l ly~ ~nrlu~l~n~ the reslstance 69 of c.,...lucLol 22 on the ~L~ limb of the circuit an,d 30 resistances 70, 71, 72 and 73 caused by the packed bed channel segments A, B, C and D respectively. The resistance 74 of the return limb of the conductor 22 is .. ~
also shown. A connection to ground is shown as 75. The t~ aLu~e controller 61 is also shown connected between the generator 20 ~,nd a - Lu. c sensing meaD,s such a3 5~3~
_ 18 --a tl , 1Q or the like, shown as 90. It will be ArprPc ;i~t~d by those skilled in the art that the t LULeS sensor 90 can -~ irate with the t~ ~ æ
c~ntrol l~r via several different means including signal 5 wires bundled with Cv~l~u. Lu~ 22.
It will also be appreciated by those skilled in the art that, in certain instances there may be no tubing 12 within the casing 8. In such ci ~ances, the cnsing itself may be used as a return co.,du~LùL in the same 10 maer as ~ rrihed above for the tubing. In this case a packer could be used to provide a hydraulic seal between the casing nnd the heater to force the solvent through the heater 30 and into the ~cuv.: ~ 20ne 15 of the reservoir.
The proper packing 50 for the present invention 15 is quite important. In the preferred ~ the packing 50 is comprised of a plurality of 8rh~r1ri~l balls.
preferred length for the heater 30 is 6 m. However, the length can vary fl~p~n~ ~ n~ on the amount of electrical power available and allowable p.~ ~ssu~e7 drop. A preferred 20 outer ~ r for the heater is that of the outer di~meter of the p~-imp, 80 the heater can then be raised and lowered onto the pump seating nipple and sealed to minimi~o fluid bypass around the outside of the heater. A
preferred inner ili Ler for the heater 30 is 4.0 cm.
25 ~owever, the inside diam~ter can vary to suit the inner diameter of the tubing in a particular well.
In a typical oilwell, the tubing 12 has a 73 mm outsr ~li; L tOD) and a 55 mm inner ~i~ LæL (ID). In a ~L~:~æLL~d ~ of the present invention, power is 30 s~rpl i~d by a 200 kW portable diesel electrical gen~tor.
The heat i~hsorhed by the solvent as it passes through the heater is calculated arcr~rrl~n~ to the following equations Q = ~T~ t - Ts jn) Cp, Den, F~
where s - 21~5~35 Q is the power dissipated in the heater (watts) Ts O"t is the solvent t~ _ Lule leaving the heater (C) T~ in iB the solvent t~ t _ ~nt~-r;n~ the heater (C) Cp, is the heat cnpncity of the solvent ( typicnlly about 5 2000 J/kg C for liquid hydlrocArhonr~ ) Den, is the density of the solvent (typic~lly about 900 kg/m~ f or a heavy ~ff' ~ ~ -te ) F, is the solvent f lowrate in m3/second Thus, for a given power or heat transfer rate, 10 higher solvent f lowrates will result in lower heater outlet t- _ Lu~ _B. Alternatively, a high heater outlet t~ can be obtained at a lower power by re-~ n~
the solvent flowr~te. Figure 1 shows thnt the required solvent volume decreases by three orders of magnitude for lS a 30 C t~ Lure rise. Thus a small t~ ~ _ rise can provide a substantial benefit in terms of reducing solvent volume requirement. However, ~8 the hot solvent is pl A~ed into the pores in the reservoir formation or rock matrix, the hot solvent will cool down and th~ rock 20 and ~ interstitial fluids will be heated. A large iraction of the cost (up to 509~) of the stimulation .-rihec1 herein is due to the cost of the solvent injected downhole. Thus, it is desirable to heat the solvent to the maximum feA~ihl~ tF', _Lu- e which avoidQ
25 solvent degradation and deleterious effects in the reservoir, such as mineral t~ l..Lo- -~ nQ. In this ~nnner a maxi~um amount of heat or thermal energy is carried by a minimum volume of solvent.
When the above formula is ~pplied to a heater 30 30 having an output power of 150 kW, and a desired t~ _ Lu e7 rise in the solvent of 200 degrees S yields a solvent flow rate of 0.42 litres per second or 25 litres per minute or 1.5 m3 per hour. As i;Qc~Qsed above, higher or lower t~ Lul~ 8 and lower or higher flowrates will 35 be ~ opliate for different solvents.

- 2155~3~

The he~t g~n~rAt i on rate within the resistive heater at steady state, is equal to the heat flux from the heater to the solvent ~8 defined in the following formulas Q= Ht A ~T
5 Where s Ht is the heat transfer coefficient between the solvent and the heater ( W/m2C ) A is the surface area of resistive heater in contact with the solvent ( m2 ) 10 8T is the local t-, Lul~ dir~eL~ e between the solvent and the heater element (C) Thus, for a desir~d solvent exit t- , LULe from the heater of 230C, (for an ~ l ,n~ e t Lu-~ of 30 C and ~ heat ris~ of 200 C across the he~ter) the 15 maximum t~ Lule: in the heater will occur in the heater element at the outlet and will be 230 + ~T degrees c-~nti~ra~i~. Thus, a resistive heater design which has a large surface area (A) and a high heat transfer coefficient (Ht) will oper.~te ~t ~ lower _- ~ for a given power and thus reduce solvent degradation.
The ~ uLa drop for a flow of 0.42 l$tre/second can be estimated by the Burke-Plummer eSrl~tinn (R.B. Bird, W.E. Stewart, and E.N. T~i~htfoot~
ILa~ .olL F` ~, John Wiley and Sons, pg 200, 1960) ôP/L = (1.75/Db"~) Dens v2 (l-~ 3 where s ~P/L is the ~L~8~U e drop per length (Pa/m) Db LI is the ball rli ~r (.003175 m) Den, is the fluid density (900 kg/m~) 3D V $8 the solvent approach velocity (0.42 m/s) is the void fr~ction (~.4 for spheres) Thus, for ~ b~ll size of 3.175 mm ~ bed length of 6 m, and flowrate of 1.5 m3/hr the pres~ure drop across 21~5~5 the heater is about 5 NPa (750 psi), which is well within the pl~suLe limitations of the tubing and lllhriratnr.
The ball size of 3.175 mm was convenient; larger balls provide less ~L~:815UL~ drop and less heat transfer surface 5 for a given heater volume while small balls result in more ~ . P.~ .~ drop and more heat tr~n~fPr surface for a given bed volume. A bed length of 6 meters is convenient however the length could vary from 1 m to 20 m ~rc~n iin~
on the particular application. The p~-~c.,u. ~ drop of 5 10 NPa, for a flowrate of 1.5 m~/hr i8 convenient however, any nfi~lrAtion with a p ~suLe drop less than 20 mPa for a f 1. .._ te greater than 1 m~/day is acceptable.
The electrical resistance of most metals iB too low to achieve any ~i~n i f i - ant heating without excessively 15 long heating .91 ~. However, in a packed bed rt~nfig~rat jcn, a high electrical resistance arises due to the limited contact are~ between ad~acent sphPri- Al balls.
The resistance of the packed bed is sensitive to a number of factors, inrl~fiin~ the amount of -~ 99ion on the 20 bed, the surface preparation and finish of the balls, the ball size, the type of metal and the maximum power applied to the bed. It is preferred to use s~hPrir Al packing P~ because the resistance will not depend on the packing orientation and the sphere to sphere contact area 25 (i.e. the resistance) will be quite uniform thL~u~ uL the bed. The accepted resistivity of Carpenter st~inl~g steel type 440C is reported to be 6x10-7 Qm. The resistivity of a packed bed of 3.175 mm balls made from the 440C steel was ~~~UrPd at 1. 6xlO-~ f2m at 45 W/cc or 30 more than two orders of magnitude higher. Thus, the resistance of a cyl in~ri~-Al packed bed 6 m long with an inner diameter of 4 cm is 0.76 n. ~rhprefore in _ well 1000 meters deep, the resistance of both legs of the co~ Lor 22 will be 2.052 for 1~4 AWG copper or 1.33~2 for 35 $2 AWG copper is 80 large compared to the heater resistance that up to 70 9~ of the power would be ` ` . 2155~35 _ 22 --dissipated in the power trAnP~ sion rather than in the heater. However, by dividing the bed into 4 8~_ ~ and connecting the F-_ ~ in series as ~ c~Psed above, the heater 30 resistance is increased by more than an order of 5 magnitude due to the reduced cross sectional area of each segment, as well as .~y the longer current path through the bed. In this manner the heater resistance is increased to lOn and the power tr~n~ inn losses are reduced to less than 17 %. Although a lOn heater resistance i8 10 convenient, a heater resistance as low as ln could be used in the present design. Higher heater resistanceg m7n7m77e the power trAnP-iL7sinn losse~ but rec~uire higher voltage~.
~he maximum heater resistance (at 150 kW) should be less than 200~ due to the breakdown of the electrical 15 insulation at high voltages.
Prom the foregoing it will be appreciated that the +- channel cnnf7~7~rAt7nn for the packed bed is not ~ nt~Al For example, an alternative r^t~ri~l for the srh~rjt i~l packing element could be used directly without 20 the "+" channel, provided it provides a packed bed resistivity of 2x10-3 S2m. Also, it will be appreciated that the ecluations set out herein can be r-n; r~ 1 Ated to change any of the parameters, such as length, power, packing element size and the like, which could yield 25 similar cnnf ~7lrAtions .
An additional benef it of the packed bed cnnfi~r~ticn arises due to the multiple electrical contacts between balls in the bed. For example each ball could be in electrical cont~ct with up to 12 ad~acent 30 balls. Thus, many p~rall~l electrical paths occur within the packed bed due to the multiplicity of electrical contacts. Because there ~re 80 many altern~te pathways for the current within a given channel segment, the packed bed heater is not prone to the burnout and catastrophic 35 fai lure problem usually associated with electrical 21550~5 resistance heaters.
It has been oLsc:Lved that the above ~ Y~rihed henter c~nfi~r~tion is self-reg~lAtin~ in that it appears to avoid excessive hot spot formation and catastrophic 5 burn out within the ~L~LeLLed power range. The ~L~f~L.~d c~nfi~lration i8 a heater with uniform grh.~r1rAl conducting el sl placed in a packed bed conFi~rAtion.
Thus each ball or conducting element is in contact with up to t*elve other conducting ~ r~n-l i n~ on whether 10 the conducting el~ment is in th~ middle of the bed or At a perimeter. The contact point between spheres is very small in cross-sectional area due to the ~:uLvntu e of the surface of the balls. Thus, the current flowing through the bed meet6 with ~i~nif~r Ant electrical resistance as it 15 passes through each contact point. This resistance, in turn, PL~nIUCe5 heat at each contact point.
When a prototype heater was tested it was observed that the bed resistance is a function of the power per unit volume. Thus, increases in power per unit 20 volume tend to decrense absolute resistance.
It was also obseL~ d that the packed bed behaves as a ~ , ~ ~ electrical resistor. For example, at 50W/cc, with various bed dimensions, the electrical resistance of the bed is inversely proportional to the 25 cross-sectional area and directly proportional to length.
Thi8 result demonstr~tes that the electrical current does not channel through the bed. This result is important because electric~l rhAnn~llin~ would create hot spots and lead to fluid r~ rZI~IAtir~n. IIOL~ L/ the bed is not prone 30 to catastrophic burnout because of the multiplicity of current pathways.
It will be appreciated that the foregoing description relates to conducting ~ which zlre - 2155~35 uniform size spheres, prefPrAhly of s~A;nlPRs steel.
However, other packed bed cnnfisr~r~tfnn~, ;
spheres of different sizes, conducting el~ R of different shapes, or Inr~ r~ conducting Pl~ ~ of S different materials of the same or different sizes or ~hapes may also be used. It is believed that the important point is to keep the bed in ~ - -; n-l ~ the contact points small between ad~acent Pl - ~, and to provide a plurality of alternate current pathways to allo~
10 the heater to ,.~ n~ an ~ l ;hri~lm which prevents local hot spot heating and the attendant burnout that may be associ~ted therewith.
In the ~.eL~ .ed method, the use of this heater conf iguration allows the solvent to be displaced through 15 a self reqn 1 ~t i ng heater which prevents catastrophic burnout of the heating element and avoids hot spot formation, and, additionally, prevents degradation of the solvent ~o be heated. This i~ important because solvent degradation could produce solid Ly~ c,.lu~Ls such as coke 20 which could plug the fluid rh~nn~lR in both the heater bed and in the oil reservoir.
Thus for 150 kW of power dissipated in the heater, the required current will be lSOA and the voltage required at the wellhead will be 1200V. The choice of 440C
25 8t~inlP~R was convenient in this application. However, many alternate materials can be substituted, ;n~ llla;r~
~etals, alloys, ceramic composite materials, ~ cnn~ tor8~ m;nPrAl R and graphite. With an alternative materi~l it may not be n~cess~ry to divide the 30 bed into sections to achieve a practical heater resistance .
The surface area of the heater element is calculated by multiplying the total number of balls in the bed by the Rurface area of a ball.

21~503~

Surface Area= (Volk,d ~ ) /Volb.~ db l~2 (1.5 1l L ID2) (1-~)/ db l, =8.5 m2 The heat LLtl~aLeL co~ff~ nt is calculated 5 using Eckert's correlation for packed beds pgs 411, 412 in T. ~ JUL L F ~ .
a~ llOOm2/m3 Go = 300 kg/m2s ~1 ~ .001 kg/ms 10 ~- 1 for spheres Re=Go/(a 11 ~)= 272.
~H=. 61 Re~ ~1~ = .061 but ~H = {Ht/(Cp, Go)}(Cp, Il/k)2/3 k ~ thermal conductivity of solvent ( .12W/m C ) 15 Therefore Ht ~ 5,000 W/m2 C
~herefore ~T = Q/Ht A = 150,000/5000x8.5 = 4 C
,h.~._f~ the maximum t~ _ _LUL~ = 230 + 4 = 234 C.
The heat transfer coeffirient in the packed bed is about 10 times better than for other conf ~ g~ration8 20 ~uch as heated tubes. In addition, the packed bed has a large surf~ce area per unit volume ~1100 m2/m3), 80 the heater is compact and has very high surface power rates t2 W/cm2) with very small temperature gradients (4 C) between the heater and the solvent. Heat transfer surface areas 25 of 10 m2 per m3 of heater volume are a lower limit of practical application. r~Pnorally it is ~ irAh]e to have as large ~ heat trlmsfer area per unit heater volume ~8 practical .
The averJ~ge r~ nre time of solvent in the 30 heater ( the void volume divided by the f l~ L~te ) ~ s 7 ~3econds. Thus the solvent heats up at ~ r~te of 30 C/second a8 it passes through the heater. The low he~ter element, LULI: and the short contact times in the packed bed are both highly desirable features to avoid solvent degradation.
A small scale heater was built and tested. A
resistivity of 1.6xlO-~ &, was ~- _ .d at 45 W/cc with AC
power with 3.175 mm rilrr~nt~r 440C stAinleAA balls at 20 5 C. This data indicates that a heater with the preferred cnnf~g~r~tlon ~I~Acrih~d herein could possibly operate ~p to 340 kW with a resistance of 12n. This result 18 more than adequate for the ~ ft~L~d design, as slightly higher resistivities require higher voltages and less ~ ge.
10 Thus, either smaller col~du~;LoLD 22 can be used or nlternatively less power is lost in ~ ARion.
It may now be appreciated how the method of the present invention may be employed. Prior to emp~oying the preferred method the pump needs to be removed from the 15 well 6. This is usually AC_ _liA~t~d by "killing" the well Nith a fluid to prevent ~ n~ d production of hydrscArhnn~ while the well 6 is open to the a _,~h~re to remove the pump. It is rr~?f~rAhle that the well be killed with an oil or solvent rather than water. However, if the 20 well has been killed with water, then the water should be displaced out of the well by circ~llA~inq oil or solvent down the annulus and back up the tubing. Once the water in the well has been ilAplRred~ a mutual solvent is preferably pumped into the tubing to further ii~plA~e 25 water away from the e.~v~Ly zone D -.v ~lin~ the 1 lhnre. A mutual solvent is a liquid which is partially soluble in both oil and water. Such a liquid is EGMBE
(ethylene glycol -'_Lyl ether) or ~A~I.Lvl,A~l/toluene.
Such a mutual Dolvent would have several b~n~fi-~iAl 30 effects, as will be now ArrreciAted. For example, the mutual solvent will increase the ~ ^hil ~ty of the solvent or oil by increasing the degree of saturation of the oil phase relative to the water phase. This mutual solvent will assist in bringing 8~ solvent 35 applications into greater contact with the wax to be - ~15~03~

treated. By increasing the degree of sat~rAtin-~ of the solvent, such a pretr~; will ~l180 facilitate the removal or ~ pl~- of the oil/solvent/wax phase from 1:he formation ~uL uu..ding the well.
The next step in the pL~ft:LL~d method i8 for the electrical cable 22 wlth the ~ars 27, resistive heater 30, and contactor assembly 32, to be lowered to the a~lu~Liate depth within the tubing 12 through the 1 ~lhr; r~trr 28 . The solvent truck 2 then begins to pump solvent into the well 6 ~t the desired rate by means of a pump 3~. As shown in Fig. 2, a hose 34 passes through the lubricator 28 down into the tubing 12 and has a nozzle 36.
It will be appreciated by those skilled in the art that the nozzle 36 may be placed at any desired 1~ r,atirr~ within the tubing 12 and in fact, it may be sufficient merely to connect the nozzle 36 to an appropriate orifice on the wellhead and simply pump the solvent directly down through the tubing 12 . Alternatively it may be des irable to connect the hose 34 directly to the heater ~e.g., if the tubing is completely blocked with wax) in order to pump solvent directly to the heater. The solvent then makes its way down the tube as in~lirAted by arrow 40 where it F~nrollnt~r~ the registive heater 30. The generator 20 i8 started and electrical power is then transmitted through 2s electrical c_ble 22 and through the tubing 12 to the heater 30. As the solvent is pumped down the tubing 12, with the valve on the annulus 10 closed, it passes through the heater 30, out the bottom orifice 16 of the tubing 12, through the pc . L~L~Lions 18, in the casing 8 and into the L~UV~Ly zone of the f ~-~n 15. In some cases it ~ay be nF~r~RsAry to seal the annulus 10 to prevent the solvent irom circulating up. In addition it may be .i.,~ i r~hle to use a packer, gelled 1,~ rl.~ or non co~ l~n~ihl~ gas to reduce heat losses due to convection in the annulus.
35 When the solvent is almost all completely - ~155035 AiF~plArP~A into the fnrr~tinn/ the power is switched off.
The conductor 22 and the hs~ter 30 and hose 34, may then be removed from the well and the well may be put back into production. Alternatively, the hot solvent may be left to 5 soak for a period of time before the well is put back into production .
In this context solvent refers to any fluid s~hich has an external phase mi~:rihl~ in all proportions with wax at the melting point of the wax. Preferred 10 solvents include crude oil and cnnA~n~atQ~ refinery distillats and L- Le cuts (nAr~th~nirl rAr~ffin~rr or i~romatic hydrocArhon~2), toluens, xylene, disgel, gA~olin~
naptha, mineral oils, chlnrinA~ed hydro~rhnn~, carbon A~ lrhiA~ and the like. MiQcihility is desirable to 15 avoid relativs ~ -hil~ty rrnhl~ as A~crih~d above.
Tn the ca8e where the solvent could be considsrsd as an l~ion (s.g., a crude oil c077~A~nin~ ~ small proportion of ~. u~uced water), then the continuous phase o~ the solvent is mi~rihl~ with the melted wax at the trsatment 20 t~ tu.~ and p ~su.e.
Ths flow rats of the solvent is dF-t~rmi --' by the pump capacity and pL~s8u~ drop across the heater, as well as ths dssirsd solvsnt t~ ~ Lu~ : rise for the available powsr supply. The depth of heat p~l.eL~aLion 25 into the formation will depend upon the total volume of solvent in~ected and the solvent t - e. The optimum distance that the heated solvent is in~ected into the reservoir will depsnd on ths amount and dspth of wax damage, as well as the porosity of the rock and will vary 30 from well to well.
Ths volums of solvsnt used ~C~OrAin~ to ths prsssnt lnvention will also vary, A~r~nA~n~ upon ths -tit~n being treated. For exampls, if ths wax dsposits or f ormation damags are prsssnt at a largs distancs away from the wellbore, then a larger volume of hot solvent will be ne~RA~y. The treatment typically will require 1-30 m~ of solvent per metre of formation being treated.
The removal of wax q lAt~on~ from the for~^tinn, or 5 even from the wellbore rods and tubing will enhance productiYlty of the well. Such wax removal will alfio enhance other types of well treatment activities, increasing the effectivenes~ of a C-~_Lu,.: tre t, an acid 8~ t i nn and the like . It will also be 10 appreciated by those skilled in the art that additives could be included in the solvent to enhance v.~rious properties. For ex_mple, these additives can include a number of chemicals, such as surfactants, dispersants, viscosity control additives, natural solvents, crystal 15 - i f i ~rR, inhibitors and the like .
As can be ArpreciAt~d from Pig. 1, increasing the t~ ~ of the solvent 30 C increases the wax carrying capacity of the solvent by 1000 fold. This L L ~ rise in turn increases the ef fectiveness of 20 the well tre~ltment and reduces the volume of liquid required. If less liquid is required, then less time is ed to pump the solvent carrying the dissolved wax out of the well, the wax iB less likely to cool down and rerre~iritate in the formation rock and the 25 oil/gas/con~ ro-te production and profitability can resume more quickly. By using a miscible heated and effective solvent, the removal of wax from pores and mi~u~or~s at the reservoir or production level can be a_ liRh~d. In the reservoir, an additional benefit of the hot solvent is 30 due to mlnimi~in~ the g~s ~nd water saturation~ and thus maintaining the highest feasible mobility or relative ?h~ y for the oil/solvent/wax phase.
The solvent is pumped or f lows through the resistive heating apparatus and is heated. For 35 convenience and ~ d r~liAhility, there may be 2~ ~5035 t~ e, pressure and flow monitoring in~L~. ~t i- n and co ltrol devices also i r~ rir~d in the heater.
It will be appreciated that this invention teaches the removal of wax deposits from oil, gas and 5 -nn~ n~lAte reservoirs and production systems by the use of a wax solvent which has been heated to greatly reduce the volume of solvent req~lired to dissolve the solid wax. The preferred method contacts the wax with a heated solvent without raising the ~i~t~rat~o~ of the water phAse and 10 re~ rin~ the mobility of the oil/solvent/wax phase. The solvent is heated near the w~x to be treated to avoid the ~ 1088 o heat (or solvent 1uid t~ ) as rihed for hot oiling.
It can now be appreciated more clearly what the 15 f~7;1in~ of the prior watsr-based heat-pro~ in~ m~thods are. In fact, it is not 80 important to apply heat to the wax to be removed, as was previously taught. It Ls much more important and effective to have a treatment which heats the solvent, and then contacts the hot solvent with 20 the solid phase wax to ~ the wax and facilitate the removal of the dissolved/melted wax from the formation before the solid phase reasserts itself. The removal of the liquid hydrocnrbon ph~se ti.e., the oil/solvent/wax phase) from the rock will be severely ob~L .luLed by the 25 ~LcgG Ict: of the water and the gcs phases due to the r~lative L -~ility effects in multiphase (i.e., water, hydrocarbon liquid, gas) flow. In other words, 7ntro~i~lr~n~ water into a f~rr-7t~ 7 has the very ~7n~7.~ir~7h7,R result of preventing the oil/solvent/wax phnse 30 from being mobile through the formation. The higher the w~ter content, the lower the ~ ty ~f the oil/solvent/wax phase. Thi~ effect is ~1 im~n~te~7, in the present invention because no water is used.
It will be appreciated by those skilled in the - 21~3~

art that the foregoing description is by way of example only, and that many variations are ro~s~hl~ within th~
broad scope of the claims. Some va~iAtinn~ have been c~!~2gPd above and others will be al/pAr~ to those 5 skilled in the art. Further, it will be appreciated that while reference has been mâde to treatment of the e:Cuv~Ly zone ., -. . o~ Ain~ a well, the method and t~ Ltll~Ufi ~r~ ;n~ to the present invention will be equally useful in removing wax damage in production 8y8tem8, i nrll-~l i n~
10 the tubing, the rods, the annulus, the wellhead, flow lines, p~re~ -, storage tanks and the like. In short, the heated liquid solvent can easily reach any wax deposits in any fluid based LL~ ' ' system. It will also be appreciated that this invention may be usefully 15 used to treat high water cut wells, or wells with water coning prohl~ -, which have selective damage to the oil siatur~ted zone due to wax. It will also be appreciated that this invention may be usefully used to treat high g~s cut wells, or wells with excessive gas production, which 20 have selective damage to the oil s~tuL~Led zone due to wax. In both water coning and high GOR (Gas Oil Ratio) problem wells, increasing the ~ hility of the oil zone by removing wax deposits can increase the production rate of oil and inrrP~ the ultimate L~CUVe:Ly of the oil from ~!i the rel~nr~
.

Claims (58)

1. An electrical heater for heating liquid comprising:
a means for attaching the heater to a source of electrical power; and a heater body having at least one liquid inlet and at least one liquid outlet; and a flow through resistive electric heating element means comprising a packed bed of conducting heating elements, the packed bed having a hydraulic pressure drop there across of 20 MPA
or less for a flowrate of 1m3/day, a heat transfer area greater than 10m2 per 1m3 of heater; and an electrical resistance of greater than or equal to 1 ohm and less than or equal to 200 ohms;
and wherein said liquid to be heated flows through said inlet, into contact with said packed bed of conducting heating elements, through said packed bed and then flows out of said outlet, and said packed bed of conducting heating elements forms a plurality of electrical contacts and thereby a plurality of alternate current pathways and said pack bed inhibits burnout of any particular electrical contact and thereby inhibits liquid degradation.

2. An electrical heater as claims in Claim 1 wherein said conducting heating elements are formed from a material having an electrical resistivity of between 10-6.OMEGA. m and 100.OMEGA.m.

3. An electrical heater as claimed in Claim 2 wherein said material is one or more of the group of metal, alloy, mineral, semiconductor or composite material.

4. An electrical heater as claimed in Claim 3 wherein said conducting heating elements are generally uniform spherical stainless steel balls.

5. An electrical heater as claimed in Claim 4 wherein said heater body completely encloses and restrains said packed bed, and said heater body includes means for applying compression to said packed bed to facilitate electrical contact between said heating elements.

6. An electrical heater as claimed in Claim 5, further including a temperature sensing means which measures the temperature of the liquid exiting the heater.

7. An electrical heater as claimed in Claim 1 wherein said heater further includes a means for adjusting the power to the heater in response to the measured temperature of the liquid exiting the heater.

8. An electrical heater as claimed in Claim 1 wherein said heater fits into a typical oil well, and has a length sufficiently long to dissipate enough power to heat a liquid solvent being passed therethrough at least 10 degrees Celsius above the temperature of the treatment area, but short enough to avoid excessive pressure (and consequent damage to the well equipment) at reasonable solvent flowrates.

9. An electrical heater as claimed in Claim 8 having a length between 1 and 20 meters.

10. An electrical heater for heating liquids comprising:
a means for attaching the heater to a source of electrical power;
a resistive heating element means comprising a packed bed of generally spherical heating elements having a hydraulic pressure drop thereacross of 20 MPA or less for a flow rate of 1m3/day;
a heat transfer area greater than 10m2 per 1m3 of heater, and an electrical resistance of greater than or equal to 1 ohm and less than or equal to 200 ohms, and a heater body containing said resistive heating element means, said heater body having a top, a bottom, and a middle for restraining said heating elements to facilitate electrical contact between adjacent elements, the top and the bottom permitting liquid flow therethrough so said liquid can contact said resistive heating element means, wherein said heater body is divided into two or more electrically insolated liquid flow channels which channels are connected in series to increase the electrical resistance of the heater, and wherein said top and bottom include upper and lower distributors having conductive portions to electrically connect said channels in series.

11. An electrical heater as claimed in Claim 10 wherein said generally spherical heating elements have an electrical resistivity of between 10-6.OMEGA.m and 100.OMEGA.m.

12. An electrical heater as claimed in Claim 11 wherein said generally spherical heating elements are made from one or more of the group of metals, alloys, semiconductors and composite materials.

13. An electrical heater as claimed in Claim 12 wherein said generally spherical heating elements comprise stainless steel balls.

14. An electrical heater as claimed in Claim 13 further including a sealing means to form a hydraulic seal between said fluid carrying conduit and said heater body to force the liquid to be heated through said heater body.

15. An electrical heater as claimed in Claim 14 wherein said liquid carrying conduit is oil well tubing, and said sealing means comprises a sealing seat which can be seated upon a pump seating nipple at the bottom of said well tubing to form a liquid tight seal therewith.

16. An electrical heater as claimed in Claim 10 further including a temperature sensing means which measures the temperature of the heated liquids exiting the heater.

17. An electrical heater as claimed in Claim 16 wherein said heater further includes a means for adjusting the power to the heater in response to the measured temperature of said heated liquid.

18. An electrical heater as claimed in Claim 1, 10 or 13 wherein said heater fits into an oil well and is sufficiently long to dissipate enough power to heat a liquid solvent being passed therethrough to at least 10 degrees above the pretreatment temperature of an adjacent treatment area, and has a hydraulic permeability sufficiently great to avoid excessive equipment damaging pressure at reasonable solvent flowrates.

19. A heater for use in stimulating hydrocarbon recovery from a well having a casing extending from above grade to an underground formation, said heater comprising:
a flow through body being sized to pass inside of said casing of said well, said flow through body including a packed bed resistance heater contained in said body, said packed bed comprising a plurality of generally rounded elements forming a plurality of alternate current pathways including a plurality of contact points between said generally rounded elements, a first electrical distributor for passing current into one end of said packed bed and a second distributor for passing current out of said packed bed; and an electrical connection for connecting said heater to a source of electrical power.

20. A heater as claimed in Claim 19 wherein said heater is adapted to be sealed to said tubing to minimize fluid by pass around an outside of said heater.

21. A heater as claimed in Claim 20 wherein said heater is sized to seal against a pump seal in said well to minimize fluid bypass around said heater.

22. A heater as claimed in Claim 19 wherein said body is formed from metal, and said heater includes an insulating liner between said heater body and said packed bed.

23. A heater as claimed in Claim 19 wherein said well includes tubing located inside of said casing and wherein said heater is generally cylindrical, and has an outer diameter smaller than an inner diameter of said tubing of said well and said heater may be raised and lowered through said tubing.

24. A heater as claimed in Claim 19, 20 and 23 wherein said heater has an electrical resistance of between 1 ohm and 200 ohms.

25. A heater as claimed in Claim 19, 20 and 23 wherein said heater body includes means for compressing said packed bed to facilitate electrical contact between elements forming said packed bed.

26. A heater as claimed in Claim 19, 20 and 23 wherein said elements are formed from one or more of the group of metal, alloy, mineral, semiconductor and ceramic composite materials.

27. A heater as claimed in Claim 19, 20 and 23 wherein said distributors are formed with a plurality of openings to permit fluid flow therethrough.

28. A portable stimulating system for improving hydrocarbon recovery comprising:
a wire line assembly for lowering a heater assembly down into a well, for providing an electrical connection between a source of electrical power and a resistive heating element in said heater assembly and for raising said heater assembly out of said well;
said heater assembly comprising a flow through electrical liquid heater having an external diameter smaller than the internal diameter of the well to be treated and being positionable adjacent a formation to be treated in said well;
a source of electrical power attached to said wire line assembly, and a volume of liquid solvent for injecting past said heater into said well, said heater heating said liquid solvent sufficiently to reduce the volume of liquid solvent required to dissolve a fixed amount of wax occurring in said well to at least one tenth of the volume required to dissolve said fixed amount of wax at a temperature naturally occurring in the formation.

29. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 28, wherein said resistive heating element in said heater assembly comprises a packed bed.

30. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29 wherein said packed bed is comprised of generally spherical heating elements have a hydraulic pressure drop there across of 20 MPA or less for a flow rate of 1m3 per day.

31. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 30 wherein said resistive heating element has a heat transfer area of greater than 10m2 per cubic meter of heater.

32. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 31 wherein said packed bed heating element has an electrical resistance of greater than or equal to 1 ohm and less than or equal to 200 ohms.

33. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29 wherein said heater assembly comprises a heater body having a top, a bottom, and a middle for containing and restraining a packed bed of heating elements, said top and said bottom permitting liquid flow therethrough so said liquid can contact said packed bed of heating elements.

34. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 33 wherein said packed bed of heating elements form a plurality of alternate current pathways.

35. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29 wherein said liquid solvent for injecting past said heater into said well comprises crude oil.

36. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29 wherein said liquid solvent for injecting past said heater into said well comprises one or more of the group of refinery distillates and reformate cuts, napthenic, paraffinic, or aromatic hydrocarbons, toluene, xylene, diesel, gasoline, naphtha, mineral oils, chlorinated hydrocarbons and carbon disulphide.

37. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29 wherein said electrical power is provided by a portable diesel electric generator.

38. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 37 wherein said system converts alternating current power to direct current power.

39. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29, wherein said system includes a temperature sensor, to sense temperature of said fluid exiting said heater, said temperature sensor communicating with a temperature controller.

40. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29, wherein said system further includes flow monitoring instrumentation.

41. A portable stimulating system for improving hydrocarbon recovery as claimed in Claim 29, wherein said system further includes pressure monitoring instrumentation.

42. A portable stimulating system as claimed in Claim 29, wherein said system includes one or more at temperature, pressure, and flow monitoring instrumentation, together with a control device.

43. A portable stimulating system as claimed in Claim 29, wherein said solvent is miscible with wax, and wherein said solvent does not reduce the mobility of any oil/solvent/wax phase present in the formation.

44. A portable hydrocarbon stimulating system comprising:

a heater assembly which may be lowered and raised in said well, said heater assembly including a flow through resistance heater having a packed bed, said packed bed being comprised of a plurality of individual resistance elements forming a plurality of alternate current pathways, by means of a plurality of contact points between said individual resistance elements, a wire line assembly for raising and lowering said heater in said well, said wire line assembly including an electrical conductor connected at one end to said heater assembly, and a source of electrical power, wherein said source of electric power is connected to the other end of said electrical conductor.

45. A portable stimulating system as claimed in Claim 44 wherein said heater assembly includes a first distributor for making electrical contact at one end of the packed bed and a second distributor for making contact at a second end of the packed bed.

46. A portable stimulating system as claimed in Claim 45 wherein said first and second distributors are located at opposite ends of said packed bed and include a plurality of openings to permit fluid flow through said first and second distributors wherein fluid to be heated flows into and out of said packed bed.

47. A portable stimulating system as claimed in Claim 44 or 45 wherein said packed bed is comprised of elements formed from one or more of the group of metal, alloy, mineral, or ceramic composite materials.

48. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said elements are generally rounded in shape.

49. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said elements are generally spherical in shape.

50. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said heater assembly includes a means to compress said elements into a packed bed to facilitate good electrical contacts between said elements.

51. A portable stimulating system as claimed in Claim 44, 45 or 46 further including a said means to compress said elements into said packed bed and wherein said compressing means is adjustable to vary the amount of compression to facilitate good electrical contact between said elements.

52. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said system further includes a temperature sensor and temperature controller for controlling said heater in response to a temperature sensed by said temperature sensor.

53. A portable stimulating system as claimed in Claim 44 wherein said system further includes a means for raising and lowering the heater in the well.

54. A portable stimulating system as claimed in Claim 44, 45 or 53 wherein said packed bed heating element has an electrical resistance of greater than or equal to 1 ohm and less than or equal to 200 ohms.

55. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said source of electric power comprises a portable diesel generator.

56. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said system further includes pressure monitoring instrumentation.

57. A portable stimulating system as claimed in Claim 44 wherein said system further includes flow monitoring instrumentation.

58. A portable stimulating system as claimed in Claim 44, 45 or 46 wherein said system further includes flow monitoring, temperature, and pressure instrumentation to facilitate control of the heater.