Formation evaluation has been greatly improved by obtaining the data from drill-stem testing and a series of logs with one trip in the hole. The log is run immediately before and after the test. Both gamma ray and resistivity curves are recorded down-hole in a self-contained apparatus. In addition to the basic information contained in the log itself, the combination with data obtained from drill-stem testing will permit more accurate and complete formation evaluation. Field examples are presented to illustrate results obtained with this technique.
Formation evaluation with dynamic techniques is an expanding technology the is putting new and useful information in the hands of the oil industry before the large financial commitments in well completion are made. The ultimate goal is total formation evaluation which could describe the reservoir size, its location, the nature of its contents and the producibility of these contents before completion expenditures are made. This goal has not yet been achieved but important progress has been made with the development of a log-test-log technique. This is an open-hole method which provides a resistivity and gamma ray log before and after a drill-stem test with a single trip into the hole. Comparison of these two logs can accurately locate the intervals which produce fluid during the test. In addition to this, both correlation information and depth information for subsequent well completion are provided. Use of log information with drill-stem test data will yield more information about the reservoir and its contents very early in the well program.
Table 1 tabulates the information that is presently available from a modern drill-stem test using multi-flow evaluation techniques. Examination of these data indicates that several additional types of information could provide much more complete formation evaluation. A normal drill-stem test can be conducted over an interval of three to several hundred feet at a depth which can be defined only by measured joints of pipe. In addition, the actual producing zone or zones are known to be somewhere within the total interval tested but cannot be pinpointed. In the case where several zones produce simultaneously, there is a need for defining the boundaries of each zone as well as the type of fluid withdrawn from each. It is also significant that 95 per cent of all drill-stem tests are run before any wireline log is run. This means that the needed log information for correlation, for verifying test interval location and selection of a good packer seat is not available for most tests. The value of drill-stem test data is increased greatly if the depth and boundaries of the producing zones can be relocated during subsequent operations. This is particularly true after casing and completion when accurate perforation and treatment can determine success or failure of the well to produce. The need for additional information has been responsible for the development of a log-test-log technique for the location of the test interval and for the definition and location of producing formations in a wellbore. Fig.1 illustrates the hydraulic changes which take place in and around the wellbore during the drill-stem test. Prior to the test (Fig. 1a) the mud filtrate has invaded the permeable formations. In the process most of the original formation fluids have been displaced, leaving around the borehole an invaded zone. During the drill-stem test (Fig. 1b) the mud filtrate is forced back into the wellbore by the flow of formation fluids. The displaced fluids collect in the borehole and mix with the mud. No reinvasion of the permeable formation takes place until a hydrostatic pressure higher than the formation pressure is built up. Nonpermeable formations are, of course, not altered by the invasion process. These important hydraulic changes provide the foundation for the new log-test-log technique.