This reference is for an abstract only. A full paper was not submitted for this conference.
The scope of this paper is to show how pressure transient analysis is used routinely in operation of an offshore field (Idd El Shargi North Dome) with long, multi-lateral horizontal completions. Occidental Petroleum of Qatar Limited (OPQL) has a Production Sharing Agreement with the State of Qatar to operate and develop ISND. OPQL works closely with technical teams from Qatar Petroleum (QP) to conduct these operations. Data is collected in the field on permanent downhole pressure gauges placed in over 75% of the completed wells. This data is primarily used to optimize gas lift systems and waterflood surveillance; an additional benefit is the use of the pressure transient information to describe complex reservoir behavior. In horizontal wells, a pressure transient passes through several flow regimes:
early-time radial flow
potentially early-time hemi-spherical flow
intermediate-time linear flow
late-time radial flow
Because of large wellbore storage effects, the early-time flow regimes are not visible; further, because of the length of time required to achieve late-time radial flow, this regime is generally not seen either. This means all data has to be interpreted off of one flow regime, intermediate-time linear flow. Using only the intermediate flow period, a non-unique solution is achieved.
Additional problems that are encountered with multi-lateral pressure transient tests include the following:
complex well geometries associated with the multi-lateral wells
large wellbore storage effects that mask much of the data
ambiguity associated with multiple solutions to the same pressure behavior
well interference effects associated with multi-lateral and nearby offsets
In addressing these problems, several "work-around" techniques have been devised. The importance of these techniques is to simplify the problem to reduce the number of variables that skew the answer. In addition to the "work-around" methods, deconvolution and rate transient analyses to improve results is routinely used. For complex well geometries, it is best to simplify the well as much as possible. In many cases, a horizontal completion can be considered vertical with a skin value of -6 to -7. This may be too much of a simplification, so a multi-lateral well with 3 individual 3000‘laterals may be analyzed as a single horizontal with a 9000’ lateral. Adding the complexity of a full triple lateral can easily distort the final solution. To overcome problems with large wellbore storage, long shut-in periods are planned in advance. When analyzing the flow volume in a multi-lateral well, a triple lateral can be equivalent to a 1000 barrel surface tank. This volume of fluid takes a long time to stop moving when shut-in occurs at the surface. The use of downhole shut-in tools is helpful but difficult to install for all pressure transients collected in the field. To overcome ambiguity, the best practice is to confine one of the variables and allow the other to be calculated. One example of ambiguity is effective permeability and effective well length. Effective well length can be assumed at some percentage of the total drilled length and the effective permeability calculated; likewise, the effective permeability can be calculated from a porosity log using a transform and the effective length calculated. Another source of ambiguity to watch for is vertical to horizontal permeability ratio and skin. It is important to bear in mind that well interference influences the pressure transients and it is often necessary to include these effects in the analysis. Another means to improve results from pressure data is to perform deconvolution on long data sets and to perform rate transient analysis. All these techniques are incorporated into routine analysis of pressure data. Pressure transient interpretation in multi-lateral wells is possible with the use of a few simplifying techniques. The solutions generated are non-unique, but they can be effectively used to understand reservoir complexities beyond the wellbore. By keeping a problem as simple as possible, the final answer is less distorted.