Method for plugging and abandoning oil and gas wells

Abstract

A method and agent to induce accelerated creep deformation of shale rock formations in the annular gap between a shale formation and non-cemented sections of a casing string have been developed. A fluid containing alkali silicate or a modified alkali silicate is added to the annular space between the shale rock formation and the casing string. The alkali silicate promotes creep deformation of the shale rock, effectively closing the annulus surrounding the casing. It has been found lithium silicate provides the strongest shale-casing bond and is the presently preferred material for closing abandoned wells.

Claims

1. A method for promoting creep deformation of shale surrounding a well casing comprising the step of introducing a lithium-based product in aqueous form into an annulus between said shale and said well casing, wherein said lithium-based product is one of lithium hydroxide, lithium carbonate, or lithium chloride, whereby said lithium-based product directly contacts the shale to promote the creep deformation of the shale, thereby sealing the annulus and creating an effective seal between the shale and the well casing.

2. The method of claim 1 wherein said lithium-based product is introduced in aqueous form.

3. The method of claim 1 wherein said lithium-based product further comprises lithium silicate.

4. The method of claim 1 wherein said creep deformation of the shale fully closes said annulus.

5. The method of claim 1 wherein said lithium-based product is added to sodium silicate.

6. The method of claim 5 wherein said lithium-based product further comprises a modified lithium silicate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 is a schematic representation showing the annular closure formed by the deformation of the shale layer surrounding a well casing.

(3) FIG. 2 is a horizontal cross-sectional schematic representation showing the annual closure formed by the deformation of the shale layer surrounding a well casing.

(4) FIG. 3 is a vertical cross-sectional schematic representation showing the annual closure formed by the deformation of the shale layer surrounding a well casing.

(5) FIG. 4 is a schematic representation of the test set-up used in Experiment 1 to demonstrate the present invention.

(6) FIG. 5 is a vertical cross section of the schematic representation of the test set-up of FIG. 4.

(7) FIG. 6 is a schematic representation depicting the operation of the test set-up of FIG. 4.

(8) FIG. 7 is a collection of photographs depicting the annular gap between the casing string and shale layer before and after the testing described in Experiment 1.

(9) FIG. 8 shows the progress of the closure of the annular gap resulting from the shale deformed by various alkali silicates used in the testing described in Experiment 1.

(10) FIG. 9 shows the results of pressure tests performed on shale deformed by various alkali silicates used in the testing described in Experiment 1.

DETAILED DESCRIPTION OF THE INVENTION

(11) The present invention provides a method for placing a fluid that will induce the deformation and creep of shale opposite subterranean rock formation. The deformed shale will have improved sealing properties due to the presence of alkali silicate such as lithium silicate.

(12) The annular deformation fluid comprises alkali silicates. These silicates can be in the form of lithium silicate, sodium silicate, potassium silicate, or a combination. The alkali silicate is usually in liquid form but can be a dissolvable solid. An example of a suitable type of lithium silicate is Lithisil®25.

(13) Suitable lithium-based products such as lithium hydroxide, lithium chloride, and lithium carbonate, may also be added to alkali silicates such as sodium silicate or potassium silicate. This fluid can impart accelerated shale creep and create an effective seal.

(14) One of the most desirable properties in an abandoned well seal is the quality of the shale-casing bond after shale creep has occurred and how much differential pressure it can hold. It has been found that holes closed with lithium silicate can withstand significantly higher pressure before re-opening than holes closed with sodium silicate. For example, in SAAB tests, it was found that although both lithium silicate and sodium silicate had similar closure times, the re-opening pressure for sodium silicate was 450 psi and the re-opening pressure for lithium silicate was 943 psi. Because it produces a stronger shale-casing bond, lithium silicate is the presently preferred material for SAAB barrier creation.

(15) The present invention will be described in connection with the following examples where lithium silicate has been used as the creep deformation agent. It is to be understood that sodium silicate and potassium silicate can also be used for other types of shale.

Experiment 1

(16) The effectiveness of lithium silicate to expedite shale creep deformation was demonstrated in dedicated, large-scale rock mechanical tests. In these tests, cylindrical shale sample (1.5 inch in diameter, 3 inch in length) from a North Sea shale formation were confined at downhole stress, pressure and temperature conditions. FIGS. 4-6 depict the equipment that was used in these tests. As shown in FIGS. 4 and 5, an artificial casing string was placed on the inside of the shale cylinder, leaving an annular gap between the rock and the casing. This set-up represents a miniaturized version of the actual situation in the field, with a casing string being left uncemented opposite a shale formation with an annular clearance that is filled in by creep deformation of the shale (FIG. 7). Using radial strain measurements and pressure pulse decay cycles, the closure of the annular gap was characterized throughout the experiments (FIG. 8). At the end of the test, the barrier quality of the shale material was probed by raising the downstream pressure and observing breakthrough on the upstream side (FIG. 9). The results that were obtained when varying the annular fluid chemistry and experimental conditions are set forth in Table 1 below:

(17) TABLE-US-00001 TABLE 1 Breakthrough Experimental Conditions Annular Closure Time Pressure Annular gap filled with artificial pore fluid, 18.4 days  Not tested temperature 55° C. Annular gap filled with 2M NaOH fluid (pH = 12), No annular closure Not observed (open temperature 55° C. observed annular space) Annular gap filled with artificial pore fluid, 11.5 days  724 psi temperature 85° C. (elevated temperature) Annular gap filled with 10% v/v Lithium Silicate 3.8 days 943 psi solution, temperature 55° C. Annular gap filled with 10% v/v Sodium Silicate 5.1 days 1054 psi  solution, temperature 55° C.

(18) The results of this experiment show that the use of alkali silicates such as lithium silicate and sodium silicate substantially reduces the time required for the shale to deform and fill the annular gap around the well casing as compared with other fluids at the same or higher temperatures. The seal that was formed with the lithium silicate and sodium silicate were stronger (with a breakthrough pressure of 943 psi and 1054 psi respectively, approximately equal within the ±100 psi accuracy of the test) than that achieved through the use of elevated temperature (with a breakthrough pressure of only 724 psi). The seal with the lithium silicate formed more quickly (3.8 days) than that with the sodium silicate (5.1 days), with the elevated temperature (11.5 days) and in the base test with artificial pore fluid (18.4 days).

Experiment 2

(19) The impact of lithium on shale was compared against conventional sodium silicate and potassium silicate using commonly drilled dispersive type shale found in Western Canada. Shale dispersion was measured by placing 30 g of the indicated shale type in a low concentration of the indicated alkali silicate that was diluted volume-to-volume with tap water. The samples were hot rolled for 16 hours at 49° C. (120° F.). After hot rolling, the samples were screened through a 16 mesh filter and rinsed with tap water. Samples were dried and weighed.

(20) Table 2 below shows the shale recovery after hot rolling.

(21) TABLE-US-00002 TABLE 2 Potassium Potassium Silicate % vol/vol Silicate (Kasil ® 1 + LiOH) Sodium Silicate Lithium Silicate Shale Name alkali silicate/water Kasil ® 1 (95:5 wt/wt) N ® grade Lithisil ®25 Shaftesbury 2.5% 47.3 62.8 39.7 35 Joli Fou 2.5% 51.7 46.0 21. 3.3 Lea Park 5.0% 48.7 10.1 6.2 0 White Spec 5.0% 11.3 49.3 6.3 0

(22) For drilling purposes, it is imperative that shales are stabilized, and hence obtaining high values for the shale recovery percentage during hot-rolling is considered advantageous. However, for the shale-as-a-barrier application, obtaining lower numbers is more beneficial, as it will lead to a larger strength reduction, and concomitantly a higher creep rate, of the shale material. It is clear from Table 2 above that lithium silicate has the lowest shale recovery percentages of the various materials tested, and is the most suitable material to induce enhanced shale creep.

(23) Various modifications and alterations to this invention will become apparent to those skilled in the art. The amount of bentonite in the thermoplastic composition can be adjusted to modify the “clump” force, suitable to avoid clumping of the pellets in any adverse storage conditions.

(24) While the above description contains certain specifics, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment thereof. Accordingly, the scope of the present invention should be determined not by the embodiment(s) illustrated, but by the appended claims and their legal equivalents.