US3372753A

US3372753A - Method of preventing hydrate formation in petroleum well production strings

Info

Publication number: US3372753A
Application number: US472598A
Authority: US
Inventors: Robert N Tuttle
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1965-07-16
Filing date: 1965-07-16
Publication date: 1968-03-12
Anticipated expiration: 1985-03-12

Description

March 12, 1968 R. N. TUTTLE 3,372,753

NG HYDRATE FORMATION IN PETROLEUM METHOD OF PREVENT PRODUCING 20M? WELL PRODUCTION STRINGS Flled July 16, 1965 INVENTOR R.N. TUTTLE United States Patent 3,372,753 METHOD (3F PREVENTING HYDRATE FORMA- TIGN IN PETROLEUM WELL PRODUCTIDN STRINGS Robert N. Tattle, Houston, Tern, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed July 16, 1965, Ser. No. 472,598 3 Claims. (Cl. 166-45) ABSTRACT OF THE DISCLGSURE The formation of hydrates and deposition of solids in a production tubing string carrying petroleum Well fluids having a high hydrogen sulfide, dissolved sulfur and water content may be prevented by maintaining the fluid pressure within the production tubing throughout the length thereof above the bubble point of said well fluids.

duction techniques that as the produced fluid flows upwardly through the tubing string certain pressure-temperature conditions may occur such that a phenomenon known as fluid flashing results. The phenomenon of fluid flashing is accompanied by a severe temperature drop which causes the sudden formation of hydrates that plug the tubing string to the extent that production is sub stantially reduced or, in many cases, completely prevented.

In most wells the aforementioned hydrates form only at the lower temperatures reached at the upper end of the tubing string. Consequently, in the past, the prevention of hydrate formation has been controlled by the application of heat to the upper end of the tubing string thereby keeping the temperature above the critical level. However, hydrates occur at much higher temperatures in fluids containing hydrogen sulfide than in the production of normal hydrocarbon fluids and, as a result, may plug the tubing string at great depths. For example, hydrates can occur at 120 F. in a fluid containing 80% hydrogen sulfide. Accordingly, prior art methods, such as the application of heat to prevent hydrate plugging, are impractical and ineffective in the production of fluids containing substantial amounts of hydrogen sulfide since there is no effective and economical way to transmit the amount of heat required to the great depths at which such plugging occurs.

Moreover, the production of fluids containing substantial amounts of hydrogen sulfide is additionally complicated by the deposition of solid sulfur throughout the length of the tubing string irrespective of the aforementioned hydrate problems associated with the production of these fluids. As the fluid containing hydrogen sulfide flows upwardly through the tubing string the pressure on the fluid decreases due to the reduction of the static head in the fluid column. It has been found that sulfur solubility decreases markedly with decreases in pressure and temperature and as a result the sulfur deposits in the tubing string as the fluid flows upwardly toward the surface. If not prevented, the sulfur deposits will accumulate and plug the tubing string.

Therefore, it is the primary object of the invention to produce a well in a manner so as to prevent plugging of the production tubing string by either the formation of hydrates and/or the deposition of solid materials out of the produced fluid.

Broadly, the invention comprises niaintaining the tubing string pressure at a value sumcient to prevent hydrate formation and/or solid deposition. Of course, it is to be understood that this maintained pressure value will vary depending upon the composition of the production fluid, etc.

More specifically, the invention comprises producing a fluid having a high hydrogen sulfide content (e.g. 50 H S or H 5 with dissolved sulfur, the balance being primarily hydrocarbons containing water or water vapor) by maintaining the pressure on the said fluid above the bubble point throughout the Well bore to prevent (l) hydrate formation (due to the lower temperature which results from flashing etc.) and (2) sulfur deposition resulting from the decreased solubility of sulfur at decreased pressures and temperatures. One particular aspect of the invention comprises the application of an external energy source, such as a bottom hole pump or compressor, to maintain the pressure in the production tubing string at or above a pressure at which hydrates form, and/or solids deposit, as determined from the pressure-volume-temperature evaluations of the produced fluids.

Other objects and advantages or" the invention will be understood from the following detailed description taken with reference to the accompanying schematic drawing.

Referring to the drawing, there is shown a well casing id extending from a location at the surface of the earth iii downwardly into a production zone such as an oilbearing formation. The lower end of casing it) has a circumferential perforations 12 to allow fiiuds from the producing zone to enter the casing 10. The casing 10 is closed at its upper end by a cover member 13. A first tubing string 14 extends downwardly from the casing cover member 13 and terminates near the lower end of the casing 10 adjacent the producing zone. A conventional sealing packer E5 is positioned in the lower end of the casing 10 to stabilize the lower end of the tubing string 1'4 which extends through a perforation formed therein. The sealing packer 15 is preferably positioned at or near the upper strata of the producing zone to prevent fluid communication between fluids being produced from the roducing zone and the interior portion of the casing 10 located above the packer 15. A second tubing string 16 extends downwardly from the casing cover member 13 to a location slightly above the packer 15 Where it communicates, through a branched connection 17, with the interior of the first tubing string 14.

A fluid operated bottom hole pump Ztl is shown as seated on an annular inwardly protruding landing shoulder 21 formed within the tubing string 1d. The outer diameter of the pump Zll is slightly smaller than the inner diameter of the tubing string 14 so that the pump may be readily lowered to the landing shoulder 21 and retrieved therefrom. To insure that the produced fluids from the producing zone do not pass upwardly around the out-side of the pump 20 when the latter is seated on the landing shoulder 21 of the tubing string 14, the pump is preferably provided with O-ring sealing members (not shown) about its circumference on both its upper and lower ends.

A power oil inlet opening 22 is provided on the upper end of the pump 20 and a power oil outlet opening 23 is provided in the side of the pump. The outlet opening 23 communicates with a circumferential groove 24 formed on the outer surface of the pump 20. The tubing string 14 is provided with a bore 25 which is located a measured distance from the internal landing shoulder 21 so that when the pump 20 is lowered into position on the landing shoulder the outlet groove 24 of the pump and the bore 25 of the tubing string 14 will be laterally adjacent one another thereby allowing spent power oil to escape upwardly through the annular space 27 to the upper end of the casing 10. The lower end of the bottom hole pump 20 is provided with an inlet opening 29 which allows produced fluids from the producing zone to flow upwardly through the inlet opening 2/ and into the lower portion of the pump 20. An outlet Opening 30, for the escape of produced fluids, is formed in the side of the pump 20 at a location adjacent the branched section 17 which communicates the tubing string 14 with the tubing string 16. The outlet opening 30 communicates with a circumferential groove 31 formed in the outer surface of the pump 20 to allow the produced fluids to flow out of the pump outlet 30 into the tubing string 16, regardless of whether or not the outlet 30 is axially aligned with the branched section 17. The pump 20 is a conventional fluid operated bottom hole pump whose operation will be more fully described infra.

The bottom hole hydraulic pump 20 is supplied with operating energy by means of a variable delivery pump 33 located on the surface 11. A power oil storage reservoir 35 is preferably provided near the pump 33, and communicates therewith through a pipe 34, to insure that the driving pump 33 may supply an adequate and uniform delivery of power driving oil to the bottom-hole pump 20.

As shown, a reduced diameter tubing 37 extends from the variable delivery driving pump 33 down through the cover member 13 of casing to allow fluid communication between the tubing 37 and the first tubing string 14. A second reduced diameter tubing 39 extends through the casing cover member 13 to allow fluid communication between the casing annulus 27 and the power oil storage reservoir 35.

A third tubing 41 penetrates through the casing cover member 13 into communication with the second tubing string 16. A choke or pressure relief valve 43 is located in the tubing 41 to insure that an adequate surface back pressure (Le, a back pressure suflicient to prevent hydrate formation and/or solid deposition) will be maintained on the produced fluid flowing upwardly through the second tubing string 16. A pipe 45 may lead to any desired facility for storing the produced fluids.

In the operation of the system the produced fluid will flow out of the producing zone up through the end of the first tubing string 14 located below the packer and into the lower inlet opening 29 of the bottom-hole pump 20. The pump is a standard conventional hydraulically operated device which is equipped with a standing valve (not shown) in the lower inlet 29 and a movable piston (not shown) located in its upper end and adapted to move downwardly when power oil is supplied under pressure through the upper inlet opening 22. As the piston moves downwardly, under the pressure of the power oil, supplied from the driving pump 33, the standing valve in the lower inlet 29 closes and the produced fluid which has entered the lower end of the pump 20 is forced out of the outlet opening into the second tubing string 16.

The pump 20 is designed in a conventional manner so that when the said piston reaches the bottom of its stroke the spent power oil flows into the casing annulus 27 via the outlet port 23 and the bore 25 of the pump and first tubing string 14, respectively, and then back to the storage reservoir via the tubing 39. The piston then returns to its uppermost position and the standing valve in the lower pump inlet 29 opens to allow more fluid from the producing zone to enter the lower end of the pump 20. By adjusting the valve 43 an adequate surf-ace back pressure may be maintained on the produced fluid to assure an operating pressure in excess of the bubble point pressure from the bottom-hole pump 20 to the surface.

Preferably, the pressure on the produced fluid in the tubing string 16 is maintained at a minimum of 200 p.s.i. above the bubble point so that the fluid will remain a saturated liquid thus preventing the formation of hydrates and/or the deposition of solids within the tubing string 16. This minimum pressure requirement of 50-200 p.s.i. above the bubble point is generally suflicient to account for the inherent fluctuations which occur in known pumping systems of the type herein disclosed. For example, the pressure is maintained at a minimum of 1500l700 p.s.i. for a fluid containing percent hydrogen sulfide.

In some instances it has been found that the bottomhole pressure at the producing zone is sufficiently high to allow production of the fluid without the necessity of supplying an external source of energy. Under these circumstances the bottom-hole pressure is suificiently high to maintain the pressure of the produced fluids above the bubble point throughout the length of the tubing string.

Therefore, a modified form of the invention comprises extending a single tubing string from the surface down to the producing zone and employing a choke or pressure relief valve, such as shown at 43, in the single tubing string at the surface. By adjusting the choke valve, the pressure throughout the tubing string may be maintained above the bubble point thereby preventing hydrate formation and solid deposition without the use of a bottomhole pumping system.

I claim as my invention:

1. In the production of an underground reservoir which produces a fluid containing hydrocarbons, moisture and over 50% hydrogen sulfide, a method of preventing hydrate formation and sulfur deposition within a tubing string extending from the surface down into communication with said reservoir fluid, said method com prising:

(a) extending a first tubing string within a well bore from the surface down into communication with said reservoir fluid;

(b) emplacing a sealing element circumferentially around the exterior of said first tubing string and into sealing engagement with said well bore at a location near the upper extent of said reservoir;

(0) extending a second tubing string within the well bore from the surface down into fluid communication with said first tubing string at a location near the top of said sealing element;

(d) positioning a hydraullically actuated bottom-hole pump within said first tubing string at a location such as to cause produced fluids from said reservoir to flow upwardly only through said second tubing string;

(e) placing a valve means within said second tubing string at the surface to regulate fluid flow through said second tu bing string; and,

(f) operating said bottom-hole pump and said valve means to maintain the pressure within said second tubing string above the bubble point of said reservoir fluid.

2. A method as in claim 1 wherein said produced fluid contains 50-100% hydrogen sulfide.

3. A method as in claim 1 wherein said pressure is maintained at least 50-200 p.s.i. above the bubble point of said produced fluids.

References Cited UNITED STATES PATENTS 371,006 10/1887 Waits. 3,223,156 12/1965 Peter 166-45 X OTHER REFERENCES Uren, Petroleum Production Engineering, 1953, pp. 601-602.

Journal of Petroleum Technology, July 1960, pp. 166l69 incl.

ERNEST R. PURSER, Primary Examiner.