Economic Optimization of Horizontal-Well Completions in Unconventional Reservoirs

Abstract

Summary Extrapolation of conventional paradigms to unconventional reservoirs can lead to disappointment and poor performance. Careful analysis of the reservoir and application of the correct stimulation design are critical when dealing with marginally economic developments. This approach includes adequate characterization of the reservoir and an understanding of the factors that control flow capacity and deliverability. One of the biggest practical problems with unconventional-stimulation-design optimization is estimating post-fracture rate, production decline, and ultimate recovery. Without a realistic prediction of the decline resulting from a given completion, it is impossible to assign value to one design over another and equally impossible to optimize the treatment for whichever goal is sought, either acceleration of recovery or increase in reserves. It is often the first—inadequate reservoir characterization—that leads to the second—unrealistic post-treatment predictions. For instance, assuming that core-derived permeability fully represents the reservoir's total flow capacity or that stimulated reservoir volumes represent the effective producing volumes can lead to incorrect diagnosis of the reservoir capability and, consequently, can lead to an inefficient treatment design. This paper presents methods for production forecasting that give reasonable post-treatment predictions that have been found useful for economic planning. The proposed methodology, backed by field observations and laboratory work, provides an economically viable plan for optimizing lateral length, fracture spacing, and treatment design. The methodology focuses on the post-stimulation effective reservoir volume. Results show that increasing apparent fracture length rarely impacts long-term recovery. Likewise, adding more fractures within the same reservoir volume may increase early-time production rate (initial production) and decline rate, without contacting more reservoir volume or adding to long-term recovery. Such practices lead to acceleration of reserves recovery, which has economic value and should be considered in the design process, but does not increase the ultimate recovery of the well once a sufficient number of contributing fractures are in place. The economically preferred completion designs may be more driven by the net present value derived in the first 5 years of production rather than the ultimate recovery of the well. This early 5-year period represents most of the useful economic life of the well, can be estimated more accurately from early performance, and is a good benchmark for completion optimization.