Abstract
Reservoir connectivity has a strong impact on the number of wells that are required to produce a hydrocarbon accumulation and, ultimately, on the associated economics of the development. Appraisal well tests are commonly conducted to confirm that wells are able to drain sufficient reserves. In a deep-water environment however, the test durations are constrained by high rig costs which consequently limit the investigated volumes to only a portion of the reservoir. This paper describes two cases where close collaboration between reservoir and drilling teams allowed to achieve long radius of investigation while minimizing rig time and risk exposure.
In the first case, a ball valve located below the production packer was acoustically activated during the final build-up to isolate the lower part of the DST string. The upper part was then pulled out of hole to carry out well abandonment. Using an electromagnetic telemetry (EM) system, it was possible to communicate with the downhole gauges installed below the ball valve to continue the monitoring of the pressure build-up during the abandonment operations.
In the second case, the rig remained on location but in a harsh environment with high risks of well disconnection. Few years ago, such operation was still considered as technically challenging with important financial exposure in case of disconnection during the DST operations. To face that challenge, the drilling team performed a careful selection and qualification of the DST string and surface equipment using the results of high angle acceptance tests and computerized finite element analysis. The DST acquisition program was optimized by the reservoir team both before and during the operations to minimize the time spent on location while collecting sufficient information. Specific forecast methods were also developed to anticipate any degradations of the weather conditions.
In the first case, it was possible to record over 600 hours of undisturbed build-up data while minimizing the time spent on location. An investigated distance of 9.5km from the well was achieved saving additional appraisal wells. In the second case, over 200 hours of pressure data were collected in perfectly safe conditions. This provided key insight on the dynamic behavior of the field and should greatly help to assess the development of this deep offshore discovery.
This paper illustrates how the combination of innovative techniques and close collaboration between drilling and reservoir teams can maximize the value of information during DSTs. This can help to reduce the number of appraisal wells and to de-risk the development of deep offshore discoveries.
