Use of Well Test Results in Oilfield Development Planning in the Timor Sea

Abstract

Summary The analysis of several transient-pressure surveys employing both analytical and numerical simulation techniques is presented. Case histories include examples of the following: partial penetration and spherical flow; flow influenced by various boundary conditions and geometries, including linear flow; variations in transmissibility; and effects from ocean tides. It is shown how these well-test results can be integrated with the geological framework, and their overall impact on oilfield development planning is discussed. Introduction The success of defining an optimum hydrocarbon-field development plan is crucially dependent on the early knowledge of the reservoir drive mechanisms and heterogeneities that may affect fluid flow in the reservoir. For offshore field developments, decisions related to the possible placement of injectors are of particular concern. As shown in the documented case histories, a greater effort to obtain quality data from extensive well testing may be warranted in such situations to confirm the geological model derived from seismic. petrophysical, and geological considerations. In any development, but particularly of significance in offshore (platform) situations, the three largest reservoir uncertainties commonly found are related to aquifer strength, sealing capacity of faults, and the extent of sand development. This is also true of the situations described here that use floating production facilities kept on location by a single-point mooring system, accommodating a small number of wells. In these more marginal field situations. there is not only a need for technically creative and novel engineering solutions that are robust and cost-effective, but also a need for a more detailed definition of future production and injection expectations and strategies. Well Testing In Timor Sea Oil Fields One new frontier in oilfield development is the Timor Sea. This offshore area, bordering the northwestern coast of Australia (see Fig. 1), has been explored to a relatively small extent, and to date only two commercial fields have been delineated: the Jabiru and Challis fields, contained in Permit AC/P4. In the Jabiru field, further described in later sections, it could be ascertained from well-test analysis that the large degree of reservoir faulting would not impede the initially indicated high production rates. A second vital piece of information obtained from well tests in this field relates to the anticipated aquifer response. In this respect, an effective vertical-to-horizontal-permeability ratio could be deduced from partial penetration and hemispherical flow. Although the well tests conducted were not strictly limit tests, they nevertheless indicated the accessibility of large reservoir volumes and the likelihood of a strong bottomwater drive. In contrast to the Jabiru field, which shows a sand development of considerable thickness, the Challis field comprises relatively thin oil-bearing sands. This field, also further described in the following sections, presented a particular challenge for well-test analysis. Because of the limited sand thickness, the impact of faults led to varied pressure-transient characteristics arising from different flow and boundary conditions. Furthermore, because appraisal wells in this field are to contribute directly to future production (subsea wells), the determination of "connected oil" is of great importance, particularly because completion costs, including costly flowlines, exceed the original drilling cost. The confirmation of reasonably sized fault compartments is also a prerequisite for the successful placement of water injectors. In the Timor Sea fields discovered so far, permeabilities are in the darcy range, requiring certain precautions to be taken during testing i.e., the use of high-accuracy gauges and tide corrections. As previously observed in Australian offshore sedimentary basins, corrections for ocean tide effects are particularly necessary, because these perturbations can be easily misinterpreted as boundary effects when superimposed on a relatively flat pressure-buildup response.