TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractLong-term data from permanent gauges have the potential to provide more information about a reservoir than data from traditional pressure transient tests that last for a relatively small duration. Besides reducing ambiguity and uncertainties in the interpretation, long-term data also provide an insight on how reservoir properties may change as the reservoir is produced. This type of long-term surveillance provides the opportunity to look at the reservoir information in four dimensions rather than obtaining a glimpse or snapshot in time. However, the installation of permanent downhole gauges is only a recent phenomenon, and a methodology for the interpretation of the data has yet to be developed. The use of long-term data requires special handling and interpretation techniques due to the instability of in-situ permanent data acquisition systems, extremely large volume of data, incomplete flow rate history caused by unmeasured and uncertain rate changes, and dynamic changes in reservoir conditions and properties throughout the life of the reservoir.This study developed a multistep procedure for the processing and interpretation of long-term pressure data. The procedure was tested with several sets of simulated and actual field data. It was found to be an effective approach for the analysis of long-term pressure data from permanent gauges.

Summary Long-term data from permanent gauges have the potential to provide more information about a reservoir than data from traditional pressure-transient tests that last for a relatively small duration. Besides reducing ambiguity and uncertainties in the interpretation, long-term data also provide an insight on how reservoir properties may change as the reservoir is produced. This study developed a multistep procedure for the processing and interpretation of long-term pressure data. The procedure was tested with several sets of simulated and actual field data. It was found to be an effective approach for the analysis of long-term pressure data from permanent gauges.

The assumption of constant rock properties in pressure-transient analysis of stress-sensitive reservoirs can cause significant error in determination of reservoir transmissibility and storativity. On the other hand, inclusion of pressure-dependent rock properties makes the governing equation for the pressure in the reservoir nonlinear. These nonlinearities can be treated only approximately by numerical means. If a permeability modulus is defined, the nonlinearities associated with the governing equation become weaker and an analytical solution in terms of a regular perturbation series can be obtained for a radial infinite-acting reservoir. Three terms in the perturbation series are derived to show the convergence and accuracy of the solution. The equation obtained for each order (zero, first, and second) in the perturbation series is solved exactly, and hence, the solution is exact to the third order.The effect of wellbore storage on the pressure behavior is also investigated. First-order approximation for bounded systems is presented to show qUalitative effects. A field example is analyzed to determine the permeability modulus and reservoir properties.

Advanced drilling technology has been widely and successfully applied to construct multilateral wells in reservoirs. This paper presents several field applications of the modeling of complex well architectures. A generalized semi-analytical segmented model, accounting for multilateral well systems in commingled layered reservoirs was used in several field applications. Cases include evaluating the flow efficiency of different configurations of wells with multiple laterals, analyzing an interference test between three horizontal wells in Al Rayyan oil field offshore Qatar, and analyzing a well test in a commingled multilateral well in a multilayer reservoir in the Dos Cuadras field offshore California. The model can predict the production performance under either constant-rate or constant-pressure conditions of a well system with any number of arbitrarily oriented laterals of any length and nonuniform formation damage. The reservoir layers, with different porosities, anisotropic permeabilities and drainage areas, are non-communicating except through the wellbore. The solution is valid for large reservoirs and when no-flow or constant-pressure boundaries affect the pressure behavior. Results of applying this method in the field cases showed that the model enabled us to predict multilateral well performance, to obtain information about reservoir connectivity, and to estimate well and reservoir properties in a multilayer system. Uncertainty, due to the large number of unknown parameters in such a complex system, represents the main challenge in using this method. It is recommended to use other means (e.g. production logging and PNC log) together with pressure transient data to reduce the uncertainty. Whenever possible, each lateral should be tested individually to provide more reliable estimates of reservoir and well properties. The presented model and the lessons learned from the field applications provide engineers a tool to use transient data collected from multilateral wells in multilayer systems for reservoir characterization and performance forecast. Background Information Methods are becoming commonplace for drilling horizontal wells, slanted wells and one or more curved wells from a common central hole in producing formations. These wells will have productivities exceeding that of a single vertical well or a single horizontal well. The performance of these wells depends greatly on appropriate reservoir selection, substantial predrilling formation evaluation and optimized completion and stimulation practices. Several studies addressed pressure transient behavior of slanted, horizontal and multilateral wells. Examples include work on spatial transformation for deviated well in anisotropic reservoir (Besson 1990), multiple horizontal wells (Economides et al. 1996, Retnanto and Economides 1996, and Yildiz 2000), and segmentation technique (Gommanard and Horne 1996). There are also studies of transient solution in layered reservoirs (Kuchuk and Wilkinson 1988, Spath, et al. 1990, and Kuchuk 1991, etc.), and field applications of multilateral well in multilayer formation (Vo and Madden 1995 and Youcef, et al. 2008, etc.) A comprehensive single- and multilateral wellbore pressure response prediction model allows arbitrary positioning of the well laterals in layered anisotropic formations. Such a flexible and generalized model can be used to study several plausible scenarios, especially the economic advantages of drilling multiple laterals from the same wellbore in a layered reservoir.