Wireline Testing Research Articles

THE FORTESCUE FIELD - NEW OIL IN THE GIPPSLAND BASIN

The Fortescue-1 well drilled in the Gippsland Basin in June 1978 was a dry hole. However, results of detailed stratigraphic analysis together with seismic data provided sufficient information to predict the possible occurrence of a stratigraphic trap on the flank of the giant Halibut structure.Three months later the West Halibut-1 well encountered oil in the Latrobe Group 16 m below that depth carried as the original oil-water contact for the Halibut field. Following wireline testing in both the water and oil-bearing sandstone units, two separate pressure systems were recognised in the well. Three additional wells, Fortescue-2, 3 and 4, were drilled to define further the limits of the field, the complex stratigraphy and the hydrocarbon contacts.Integration of detailed well log correlations, stratigraphic interpretations and seismic data indicated that the Fortescue reservoirs were a discrete set of units stratigraphically younger and separated from those of Halibut and Cobia Fields. Analysis of pressures confirmed the presence of two separate pressure systems, proving none of the Fortescue reservoirs were being produced from the Halibut platform. Geochemical analysis of oils from both accumulations supported the above results, with indications that no mixing of oils had occurred.Because the Fortescue Field is interpreted as a hydrocarbon accumulation which is completely separated from both Halibut and Cobia Fields, and was not discovered prior to September 17, 1975, it qualified as "new oil" under the Federal Government's existing crude oil pricing policy. In late 1979, the Federal Government notified Esso/BHP that oil produced from the Fortescue Field would be classified as “new oil”.

WIRELINE FORMATION-TESTER PERFORMANCE ON THE NORTH WEST SHELF

The formation-interval-tester was introduced to the North West Shelf in December, 1970 Originally it was used as a first-look test method, with drill stem testing retained as the standard evaluation technique in all potential reservoir sections. Systematic use of this technique in parallel with drill-stem testing has allowed the accumulation of a significant volume of comparative data. This has provided a unique opportunity to examine the validity of wireline-testing as an alternative to the more traditional method of reservoir evaluation.The wireline-tester has been found to be a reliable indicator of movable hydrocarbons in reservoir rock, after experience allowed the development of regional interpretation limits. Such variables as gas:oil ratios and oil gravities are determined within acceptable limits of accuracy. Formation pressure determinations are consistent' and are thought to be more reliable than those derived from drill-stem-testing. With the data available it is normally possible to calculate the indicated formation permeability. This however being a single point determination the result is of questionable value. Experience has shown that "order of magnitude" agreement is normally achieved between wireline-tester permeabilities and those determined from drill-stem-testing in the same interval.The reduction in cost resulting from the application of this technique is particularly significant. Savings may be related directly to the relative reduction in rig time required by this operation compared with that required for drill-stem-testing. As the cost of offshore operations increases rapidly the resultant savings will grow in significance.Also of concern in all well-testing operations is the risk to which the rig and personnel are subjected. On floating offshore rigs the flowing well condition required by drill-stem-testing requires the acceptance of certain associated risks. Wireline testing is by comparison a risk-free operation.The recognition of the inherent limits of this approach to well testing is important if satisfactory results are to be achieved. Where the required reservoir parameters are those that can be satisfactorily determined with the wireline tester it offers a real alternative to the more traditional evaluation techniques.

Theoretical Analysis of Pressure Phenomena Associated with the Wireline Formation Tester

Abstract The pressure build-up technique is a recognized method of determining permeability from conventional drillstem tests. In this paper an effort is made to extend such techniques to the interpretation of data obtained from the wireline formation tester. Such a study is necessary because of the differences, for this case, in the magnitude of the flow parameters (rate of flow, amount of recovered fluids) and in the flow geometry (flow through a perforation vs flow across the face of the wellbore, etc.) involved in the solution of the equations of flow for compressible fluids. The perforation is replaced by a spherical hole, and the effect of the borehole is neglected, so that the flow can be considered to be radial in a spherical co-ordinate system. Arguments are presented to justify this idealization. Assuming single-phase flow, general relations between pressure and flow rate are developed for a homogeneous medium. The study is then extended to permeable beds of finite thickness. It is shown that the early stages of pressure build-up tend towards spherical flow, while the later stages tend towards cylindrical flow. The thinner the bed, the more quickly flow approaches the cylindrical model. The prevalence of thin beds in practical work makes this analysis quite important. Cases involving permeability anisotropy are treated. Introduction From wireline formation tester operation, two types of data are obtained:the nature and amount of recovered fluids, andthe pressure history recorded during the test. A number of papers have been written dealing with the interpretation of formation production on the basis of the recovered fluids. In general, the methods described have been quite accurate for both high- and low-permeability formations. The present paperwill deal with an analysis of the pressures observed. An analysis of the pressure build-up curves obtained in hard-rock country has already been attempted on the basis of the formula proposed by Horner. Although this approach has met with success in many instances, some questions have been raised as to its validity. It is the aim of the present study to place the analysis of pressure build-up in the formation tester on a firmer basis, from which more detailed methods of interpretation can evolve. Because of the great differences between the operation of the wireline formation tester and the conventional drillstem test, modifications are necessary in the interpretation. The major difference relates to the flow geometry. Once the flow geometry has been established other features such as multiphase flow, skin effect, afterflow, etc., well described in the literature, can he introduced. It will be assumed that the mechanical operation of the formation tester is already known to the reader. It will suffice here merely to state that the tester provides the means for taking a relatively small sample of the fluid immediately adjacent to the borehole, and for recording the subsequent pressure response. In comparison with conventional drillstem tests, the time required for a satisfactory pressure build-up response is much shorter, because of the relatively small quantity of fluid withdrawn by the wireline tester. This feature is highly desirable in the case of low-permeability formations. For an analysis of the pressure response within the formation, three simple flow geometries are considered-linear, cylindrical and spherical. The spherical and cylindrical flow geometries are most pertinent to the formation tester; therefore, they will receive the major emphasis. Since the configuration of the borehole and the perforation made by the tester complicate the flow geometry, it is necessary to allow for them in the drawdown response. However, because of the volume of formations contributing to the pressure- response, the details of the perforation shape are unimportant in the build-up period. Since relatively small amounts of fluid are withdrawn from the formation, in contrast to a conventional drillstem test, a study of the "depth of investigation" and the significance of drawdown as well as build- up data will be included. Because the "depth of investigation" will be shown to be rather large, the effect on the build-up curves of the finite thickness of the permeable bed is considered. It is this consideration that leads to the importance of cylindrical flow geometry. Also included is a discussion of permeability anisotropy and its effect on the interpretation of the tester results. The pressure curves recorded by the formation tester will follow two general patterns, depending upon whether the formation is of high or low permeability. Fig. 1 (a and b) schematically illustrates these two responses. In Fig. 1(a), the high pressure recorded during fill-up of the tool is essentially the pressure differential across the choke in the system. JPT P. 899^

Formation Evaluation with the Wireline Tester - Merits and Shortcomings

Formation Evaluation with the Wireline Tester - Merits and Shortcomings

Formation Evaluation with the Wireline Tester - Merits and Shortcomings

Abstract The wireline formation tester has proved to be a valuable tool for formation evaluation, but some tests still lead to misleading interpretations while others are inconclusive. Longer flow periods and more tests are generally required for evaluation of low-permeability sands. No particular advantages or disadvantages were found for tools with large, small and/or segregated chambers. The methods of interpretation presently in use have proved to be quite accurate; however, care must be exercised to determine that the conditions required for use of these methods are satisfied and that the tool functions properly. Errors in the measurement or recording of recoveries have probably resulted in many misleading interpretations; simply stating all volumes in the same units would speed interpretation and be helpful in recognizing errors. A single chart for interpretation of formation test recoveries from all sizes of chambers is presented. The surface pressure is helpful in the detection of a leaky seal, but it should not be used to correct for losses caused by leaks. Water resistivity data may be misleading when used to predict whether a zone would produce water and hydrocarbons or water alone. The wireline formation tester is a dependable tool for obtaining pressure measurements; however, the estimation of formation permeability from pressure build-up has been found to be qualitative at best. Introduction The wireline formation tester was introduced to the industry in 1955 by the Schlumberger Well Surveying Corp. It proved so successful that thousands of tests have been run to date and most logging companies now offer this service. It is a valuable addition to the array of wireline tools currently being used for formation evaluation and is the only one, other than the side-wall sampler, that obtains physical evidence of formation contents.