52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
2 in the last 30 days
165 since 2007
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ABSTRACT: In many oil and gas wells, long sections of the annulus between casing and rock are left open during completion. These are potential pathways for leakage, and may need to be sealed off in critical places. In numerous field cases in shale formations it has been found that the rock has crept in and closed the annulus by itself. If this happens, it represents huge cost savings for the operator. In order to study this process under controllable conditions, we have designed a specific laboratory test where shale samples are subjected to similar conditions as the rock around a well, and their ability to establish sealing barriers are tested. The test reveals how the sealing efficiency of the barrier and the load on the casing develops with time. Post-test μΟΤ scans reveal information about the permanent structural changes the rock has suffered during the shale barrier forming process. The test results can not be directly transferred to field conditions for a specific rock, however.
Leakage along oil and gas wells can be a problem with potentially serious consequences. Prior to drilling, the hydrocarbons are trapped below thick layers of tight, sealing shale. The wells are breaking this seal. In many wells, long sections through the overburden are completed with an open annulus, thus providing an efficient pathway for leakage unless blocking barriers are placed within the sealing layers. This problem has to be fixed at latest when the well is going to be plugged and abandoned.
Field experience has shown that shale in some cases creeps in and closes the annulus (Williams et al., 2009). In other words, nature itself takes care of the problem. This may represent huge cost savings for the operator. However, in many cases the self-sealing process do not occur. Lack of knowledge about the process has made it difficult to predict whether it will happen, and to design well completion such that the process is promoted.
In order to better understand the self-sealing process, we have developed a laboratory test which is intended to simulate the field conditions during the process. The objective of such a ”Shale Barrier Test” is to investigate whether a specific shale material is able to form a sealing barrier around a casing, under a given set of conditions with respect to stress, pore pressure, temperature, fluid exposure, etc. By comparing the behavior of different rocks, it is possible establish some criteria for which rock properties are favorable and which are unfavorable for the formation of a shale barrier. The results from a specific test can not be directly transferred to field conditions for the same rock however, as discussed in chapter 6.
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