--> Reserve Evaluation by Geophysical Methods: A Case History from China, by J. B. Gustavson and S. Xin; #90986 (1994).

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Abstract: Reserve Evaluation by Geophysical Methods: A Case History from China

John B. Gustavson, S. Xin

When investing in developed international oil and gas properties, the oil industry faces numerous risks. It is therefore important to reduce the geological and reserve risk as much as possible to be able to accept the other unavoidable political, market and economic risks. Consequently, investment decisions must be made based on only a limited number of well penetrations; therefore, it is of prime importance to derive as much reserve data as possible from the well penetrations and from available seismic data. In short, the geophysical data provide an opportunity to obtain a broader lateral measure of the reserves than just the spacing around each well penetration.

In 1989, an integrated reservoir study was completed of the Suizhong 36-1 oil field in the Bohai Gulf, China. Now, after several years of development drilling, the predictions of the original study can be compared with the actual results, and important conclusions can be made with regard to the applicability and accuracy of this geophysical reserve evaluation method.

A total of 52 wells have been drilled with data being directly applicable to the new detailed mapping of the reservoir. Upon evaluation of full suites of logs from these wells, the reservoirs have been remapped and the reserves have been adjusted. The in-place reserves increased due to a combination of several factors. First, the original study was conducted with a certain conservatism in the interpretation of values between well penetrations. Second, the use of stringent cut-off limits provided a conservative result in the first study. Well testing such as DSTs on only a very few wells provided data more conservative than the results from the numerous development wells.

Not only were the in-place reserves increased through the development drilling, but also the recovery rates estimated by simulation studies were increased. The reason for this is that a larger number of smaller and more accurate cells could be used in simulations. The combination resulted in an overall increase in reserves, although the question of whether these reserves are proven, or some of them only probable, still remains a point of discussion among geologists, geophysicists and engineers.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994