| dc.description.abstract | Shale swelling causes 90% of wellbore instabilities and oil-base muds (OBM) have been the best choice for many years. However, the stringent environmental concern have resulted in a keen interest in the development of highly inhibitive, high-performance water-base mud (HPWBM). Recently, polyamines have been widely studied as high-performance shale inhibitors and applied around the world. However, the results indicated that the polyamines were incompatible with bentonite. In this study, we clarified the incompatibility issue. Then, we checked the performance of lignosulfonate commonly used in the industry as a deflocculant and found it to be insufficient. For further improvements, we introduced polyvinyl alcohol (PVOH) as a new polymeric deflocculant and found that PVOH worked better than lignosulfonate. Shale swelling has been commonly measured using linear swell meter (LSM). Since preserved shale core is rarely available, swelling tests are usually performed on “pellets” created from cuttings. Even though capillary pressure effect can be avoided by ensuring full saturation of the pellets, they would swell more than the intact shales. Besides, the LSM measurements are performed under atmospheric pressure and temperature, whereas downhole swelling obviously occurs under in situ conditions. For these two reasons, they do not represent the actual downhole swelling and thereby we are not able to predict wellbore stability. Some studies have been done on the measurement of swelling under high pressure, however, they all require specially designed equipment. In this study, we measured swelling of both pellet and intact core samples under various stress conditions using the conventional LSM in order to estimate the downhole swelling by extrapolation.
And we analytically checked the consistency of the swelling measured under atmospheric pressure and the one estimated by extrapolation with field observations. As a result, we found that prediction of the wellbore stability could significantly be improved by taking the effect of stress into account. Then, we proposed an improved method that could analytically predict the wellbore stability from the estimated downhole swelling even when the preserved intact core was not available. | |
