CA2120098A1 - Process for controlling gas influx into well bores - Google Patents

Abstract

A process for controlling gas influx into a section of a well bore is provided. The invention comprises placing a squeeze fluid in the well bore and maintaining an applied pressure to the squeeze fluid so that the squeeze fluid is forced radially into permeable formations of the well bore wall to create a flushed zone of reduced permeability to gases. Additional embodiments of the process involve the additional steps of displacing the squeeze fluid remaining in the well bore to the surface by a drilling solution, adding a casing to the flushed zone thus creating an annulus between the casing and well bore wall and subsequently pumping a cement slurry into the annulus to seal the annulus between the casing and well bore wall. The process of the invention may be applied during primary drilling or at a remedial setting.

Description

. 212U~98`

Field of the Invention The present invention relates generally to well drilling procedures and more particularly to a method of stopping low volume, low pressure gas influx into freshly placed cement during primary drilling operations or stopping low volume gas influx in existing wellbores (remedial setting).

Background of the Invention Gas migration problems have plagued oil and gas well bores for several decades. It has been found that gas migration or leakage from abandoned well bores have resulted in damage of farm lands. of more recent concern is the presently unknown contribution to greenhouse effects by the release of methane gas from these abandoned well bores. Consequently these environmental and economic concerns have resulted in an increasing need to repair existing leaking wells as well as preventing newly drilled wells from subsequently leaking gas.

Gas migration problems generally originate during the initial stages of "setting" of cement, used in casing wells. During this stage gas bubbles may migrate upward through the unset cement slurry, the interface between the cement and the newly drilled well and between the casing and cement. This may create permanent channels or conduits after the cement has set.
These channels or conduits permit communication between the well bore face and the surface. As a result, many difficult and expensive well control and production problems as outlined above, occur.

Several attempts at stopping gas influx or 212Q~8 _ -- 3 migration into freshly placed cement have been tried over the years. One of the mechanisms which causes gas influx originates during drilling operations wherein hydrostatic pressure imposed by drilling fluids on gaseous intervals is greater than formation pressure, therefore, gas is held back from entering the well bore and escaping. The well is drilled to a so-called total depth (TD) and evaluated for economic purposes. If it is found to be economically feasible production casing is run and subsequently cemented in place. Several theories as to what happens to the cement, after placement, with regards to gas influx, is postulated.
One of the theories holds that as cement "sets up", that is as gel-strength develops and shrinkage occurs due to hydration, the ability of the cement to effectively impose pressure (hydrostatic) on gas containing permeable or porous media is comprom`ised.

Eventually, hydrostatic pressure degrades to a point where pressure inversion occurs (i.e. the gas pressure in the formation exceeds the hydrostatic pressure cited by the cement). At this point, gas at a well bore face is no longer held in place by cement hydrostatic pressure and therefore may move into fresh or "green" cement and take several leakage paths to the surface, resulting in the gas migration problems.

The time during which cement changes from a liquid to a solid is termed "transition time". Attempts have been made to control cement gel strength development, that is reducing the transition time to lessen chances of gas influx into the cement.
Fundamentally however, developing cement slurries with zero transition time has not been achieved. Further additives which either coat the invading gas bubbles (surfactants) or cause cement to generate gas within 2~ 20098 itself have been tried with varying amounts of success.

Several other techniques are readily available with regard to controlling gas influx. These include running tools which either isolate gas bearing intervals by mechanical means such as packers or by running sacrificial strings of tubing which provide a conduit for gas to be diverted therethrough.

Other obstacles to obtaining successful primary cement placement in areas where gas influx is prevalent are porous or permeable zohes with no history of "economic" gas potential and hence are extremely difficult to pinpoint. Therefore identifying zones which require treatment at a primary stage of the drilling operation is virtually impossible and at a remedial stage:is both costly and not always successful.

For example, U.S. Patent No. 1,842,106 describes a method and apparatus for cementing of oil and gas wells which attempts to avoid the deleterious action or dilution of the cement by the action of gas.
The method of this patent relies entirely on generating and directing artificial pressure at the surface and transmitting this pressure through special mechanical apparatus to previously placed cement slurries. This method has disadvantages in that it does not address alteration of permeability to gas adjacent to the well bore. It further assumes production casings have been run and attempts are now made to cement it in place.
Finally, this patent does not elude to or even suggest controlling gas influx in upper hole shales or stopping gas influx during primary drilling operations.

2~2~98 -In United States Patent 4,600,056 a method and apparatus is described which is particularly suited for completing wells such as gas wells at depth such that the pressure exceeds the burst strength of conventional external casing packers. In an embodiment of the invention cement is introduced into an annulus between a 5-1/2" and a 9-5/8" casing. A predetermined volume of cement is introduced and is moved down the annulus until the bottom of the cement is at a desired point above a predetermined cement port. During these operations, gas continues to flow through a tube. The cement is allowed to set until it has reached a predetermined hardness.
The foregoing procedures allow cement to be introduced and set with no contact between the cement and deleterious gases at the high press`ures prevailing in the well. After the cement has set, the flow of gases through the tubing is stopped by shutting in the well.
The cement in this invention is not contaminated by gases as the invention utilizes various strings of pipe to allow high pressure gas a conduit or path to escape to the surface while the cement is placed in prospective intervals. The above process does not mention or even suggest altering the physical or chemical characteristic of a well bore to control gas influx. Furthermore, this process is aimed at high volume, high pressure cementing of gas wells at a completion phase and does not deal with gas influx at a primary non-cased drilling phase.
Finally the entire process of this patent relies on mechanical packers and strings of casing being properly placed in a high pressure, high volume gas well.

United States Patent 5,020,594 describes a method of cementing wells to minimize gas intrusion by applying pressures in the subsurface formations, substantially equal to the natural formation pressures.
The invention is based on a mechanism termed "ballooning" and attempts to minimize gas intrusion while reducing design density of the cement slurry when a ballooning situation is encountered. Therefore, it may be seen that this patent seeks to provide a solution whereby control of gas influx rests primarily with matching slurry density with formation pressure.

As may be seen from the above discussion, efforts to control gas influx (at a primary level) into fresh cement have relied entirely on either cement composition or placement techniques or a combination of the two. The applicant is unaware of attempts at controlling gas influx prior to cementing the well and indeed prior to running casing into the well bore.
There is therefore a need for a process for reducing or eliminating gas influx into new cement which overcomes disadvantages of the prior art and which is based on the fact that distinct intervals which may introduce gas to a well bore can be virtually impossible to identify therefore a solution is sought which may be capable of treating the entire well bore simultaneously without having to determine the problem intervals. It is also desirable for a process whereby knowledge of the "gas source" is unnecessary for success of the process.
Furthermore the desired process must be applicable to both abandoned and newly drilled wells.

8ummary of the Invention The present invention seeks to provide a process for controlling gas influx into a well bore prior to cementing the well bore.

In providing a process for controlling gas influx into a section of a well bore, the invention comprises the steps of:

2~8 a) placing a squeeze fluid in the well bore;
b) maintaining an applied pressure to the squeeze fluid so that the squeeze fluid is forced radially into permeable formations of the well bore to create a flushed zone of reduced permeability to gases.

A further embodiment of the process further comprises the step of displacing remaining squeeze fluid in the well bore to the surface by a drilling solution.

A further embodiment of the process further comprises the steps of adding a casing to the flushed zone to create an annulus between the casing and well bore wall and pumping a cement slurry into the annulus to seal the annulus between the casing and well bore wall.

Brief Descripti.on of the Drawings The invention is illustrated by way of example in the accompanying drawings in which:

Figure 1 is a schematic cross section of an upper portion of a well bore;

Figure 2 is a schematic cross section of the same well, filled with a squeeze fluid;

Figure 3 is a schematic cross section of the same well with a pressure being applied to the squeeze fluid;

Figure 4 is a schematic cross section of the same well after the squeeze fluid is displaced; and Figure 5 is a schematic cross section of the ~l2n~s ` -same well after drilling operations are resumed.

Detailed Description of a Preferred Embodiment Referring to Figure 1, a well bore is generally designated by numeral 10. The well bore 10 has bore hole walls 12 which may extend through various formations such as shale formations, gas bearing formations, and fluid bearing sand formations. The well bore 10 is drilled to a predetermined depth Dl, and a tubular casing 13 is run down the well bore 10, leaving an annular space between the bore hole wall 12 and the annular side of the casing 13. The annulus is cemented off using conventional cementing techniques forming a cement casing 14, which are well known in the art. This cemented section D1 may or may not have been treated according to the process of the present invention.

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For the purposes of understanding the process of the present invention the bore hole 10 is shown drilled to a further depth D2 beyond the upper cemented section Dl. The drill string (or dual pipe) 16 is left in the bore hole 10 which has been drilled to a depth D2. Once the bore hole 10 has reached this pre-determined depth D2 drilling operations are suspended with the bore hole 10 filled with drilling fluid or mud 18. It must be remembered that drilling operations should be suspended prior to reaching any interval for which damage is to be avoided. The drill string (or dual pipe) 16 is left in the hole and a pre-determined squeeze fluid 20 of appropriate viscosity and volume is pumped down the inside of the drill string 16.

The squeeze fluid 20 displaces the drilling mud 18 in the annulus between the drill string 16 and 2~0098 -g the bore hole wall (Figure 2). The drill string is then withdrawn to the depth Dl of the cemented section 14 and the bore hole 10 is packed off to the atmosphere using an annular preventer, pipe ram or other suitable sealing means 22 (Figure 3). A pressure is then applied from above (drill pipe side, annular side or both sides simultaneously) to the squeeze fluid 20. In an alternative embodiment, the drill string 16 is maintained at depth Dl when pressure is applied.

As indicated by the arrows extending radially into the bore hole wall of figure 3, a pre-determined volume of squeeze fluid 20 is forced into the formation by the applied pressure. After the pre-determined volume of squeeze fluid has been forced into the formation, the pressure is released and the rams or preventers 22 are opened. The remaining squeeze fluid 20 in the well bore 10 is displaced to the surface by drilling fluid 18 pumped down the drill string 16 (Figure 4). Drilling operations may then be resumed below the newly created flushed zone 24 (Figure 5).

Further flushed zones may be created below the upper flushed zones. These may then be later cemented during the drilling phase and the process may be repeated for remaining formations.

In determining the type of squeeze fluid composition to be used, a determination of the formation characteristics of the bore hole must be made. For example, a low permeability rock formation requires a squeeze fluid of low viscosity whereas for formations of high permeability, a high viscosity squeeze fluid may be used. It has been found that inhibited water may be used effectively in varying situations such as an A12 (S4) 2 system. Another potential squeeze fluid is - lo - ~ 309~
Dowell Schlumberger's commercial product Permabloc~ with a viscosity of approximately 1 cp. A range of squeeze fluid viscosity would typically range from 0.85 to 5.0 cp .

It must be remembered that at all times the injections pressure (hydrostatic pressure plus gauge pressure) must be less than formation fracture pressure (typically 18.1 kPa/m).

Factors that determine the pressure range are squeeze fluid viscosity and rock permeability. The fracture pressure of the formation is established from published formation fracture data, by performing leak-off tests or by computer analysis. This information is well known and will not be discussed further.

In order to perform the process of the subject invention, it is necessary to determine a rough or approximate figure for the permeability and porosity of the design interval to be squeezed. This is determined during the drilling of the formation from the drilling penetration rate and the quality of material returning to the surface. A high penetration rate usually indicates a highly permeable/porous formation and the quality of the material returning to surface may provide an indication of the porosity of the formation. These factors are readily determined by persons skilled in the art. As explained above, once the permeability is known, the viscosity of the squeeze fluid may be determined. The porosity, depth of the interval and desired radial flush distance determine the maximum volume of squeeze fluid which may be squeezed into the formation. Once the above factors are determined, the process may be implemented as described earlier.

3~8 The flushed zone process was tested in a number of wells. An analysis of the results shows advantages of using a squeeze fluid in reducing gas leakage from wells. These tests are described more thoroughly in the inventor's paper "Flushed zone process helps control gas migration in primary cementing", Oil and Gas Journal, August 16, 1993.

Example 1 Well 6-27-49-4W4M (6-27) was drilled with an Al2(SO4)2system and subsequently abandoned. The main hole was drilled to 500m measured depth, and then a resin squeeze was performed. The Al2(SO4)2 system was displaced from the well with a Permabloc~ fluid. Approximately 70 1 were squeezed away at 1,600 KPa surface pressure. This volume equates to 1.08 linear meters of porous silt/shale with a porosity of 10% being flushed to a radial distance of 75 mm. Once the squeeze was performed, the well was displaced with original drilling fluid, and drilling operations resumed. The well was subsequently abandoned. While the well is leaking gas, the leaking did not start until roughly 8 days after abandonment plugs were run. As the original design of the flushed zone theory was to avoid contamination of the cement slurry by supplying an additional 16-48 hours of lag time before the gas migrated back to the well bore face, the initial objectives of using a sgueeze fluid were realized.

Example 2 Two other wells (Well #1 (11-9-53-24W3M) and Well #2 (13-25-52-24W3M) were drilled according to the following drilling program:
Hole Size 222 mm 8-3/4 in.
1st Interval: Well #1 - 140m to 283m ~:~ 4~8 Well #2 - 140m to 307m Mud Type: 3~ Ammonium Sulfate/XC Polymer/Calcium Carbonate 2nd Interval: Well #1 - 283m to 620m Well #2 - 307m to 629m Mud Type: Gel Chem Procedure:
Drilling fluid was circulated through the rig system. The shoe was drilled out with water and the cement contaminated water was discarded prior to displacing to the (NH3)2S04 system. The well was drilled ahead maintaining the mud system. Fluid density was raised to and maintained at 1130 - 1150 kg/m3 with additions of CaC03 and XC (xanthum gum) polymer. Key offset showed pressures of 2770kPa at 250m Total Vertical Depth (EMD = 1129 kg/m3).

After reaching the lst interval TD's, the (NH3)2S04 system was displaced with Permabloc~ using Dowell Schlumberger tank/pumping units. The (NH3)2S04 system was caught at the surface on well #l and moved to well #2 for subsequent re-use. The wells were squeezed at 4525 kPa bottom hole pressure (pressure limited) and approximately 70 to 100 litres of Permabloc~ were squeezed away. After the wells had been displaced and "squeezed", the residual Permabloc~ was circulated out of the hole with a standard Gel Che~ mud system (i.e.
low Plastic Viscosity, Yield Point, water loss = 12cc, density = 1130 - 1150kg/m3). Drilling was continued to TD with a Gel Chem system and logs were run.

These wells were subsequently cased and have not shown signs of leaking.

While the invention has been described in 2~.2~

connection with specific examples, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

The terms and expressions which have been employed in the specification are used as terms of description and not of limitations, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claims to the invention.

Claims (18)

1. A process for controlling gas influx into a section of a well bore comprising the steps of:
a) placing a squeeze fluid in the well bore to cover the section of the well bore;
b) maintaining an applied pressure to the squeeze fluid so that the squeeze fluid is forced radially into permeable formations of the well bore wall to create a flushed zone of reduced permeability to gases.

2. A process as in claim 1 further comprising the step of displacing squeeze fluid remaining in the well bore to the surface by a drilling solution.

3. A process as in claim 2 further comprising the step of adding a casing to the flushed zone to create an annulus between the casing and well bore wall.

4. A process as in claim 3 further comprising the step of pumping a cement slurry into the annulus to seal the annulus between the casing and well bore wall.

5. A process as in claim 1 where the squeeze fluid is added through a drilling tubulars withdrawn to above the flushed zone prior to maintaining the applied pressure.

6. A process as in claim 1 where the squeeze fluid is added through a drilling tubulars maintained within the flushed zone prior to maintaining the applied pressure.

7. A process as in claim 1 where the volume of the squeeze fluid added to the well bore is determined by the permeability of the formation.

8. A process as in claim 1 where the volume of the squeeze fluid added to the well bore is determined by the desired radial flush depth.

9. A process as in claim 1 where the applied pressure is determined by the fracture pressure of the formation.

10. A process as in claim 1 where the viscosity of the squeeze fluid added to the well bore is determined by the permeability of the formation.

11. A process as in claim 1 where the applied pressure is maintained for 1-3 hours.

12. A process as in claim 1 where the applied pressure is about 18.1 kPa/m.

13. A process as in claim 1 where the squeeze fluid is water.

14. A process as in claim 1 where the squeeze fluid is Permabloc?.

15. The process of claim 1 where said process is performed during primary drilling of the well bore.

16. A process for controlling gas influx through a section of a an existing well bore wall comprising the steps of:
a) drilling a plurality of sealing drill holes around the perimeter of the existing well bore wall;
b) adding a squeeze fluid to the sealing drill holes;
c) maintaining an applied pressure to the squeeze fluid from above so that the squeeze fluid is forced radially into formations of the well bore wall to create a flushed zone of reduced permeability to gases.

17. A process as in claim 16 further comprising sealing the sealing drill holes with a cement plug.

18. A process for controlling gas influx into a section of a well bore wall of a well bore during primary drilling comprising the steps of:
a) placing a squeeze fluid in the well bore;
b) applying a pressure of about 18 kPa/m for 1-3 hours to the squeeze fluid from above so that the squeeze fluid is forced radially into permeable formations of the well bore wall to create a flushed zone of reduced permeability to gases.
c) displacing remaining squeeze fluid to the surface by a drilling solution;
d) adding a casing to the flushed zone to create an annulus between the casing and well bore wall;
e) pumping a cement slurry into the annulus to seal the annulus between the casing and well bore wall.