Pressure build-up analysis of Devonian shale gas reservoirs is very critical and essential for determining the reservoir formation properties such as the flow capacity (Kh), skin factor (S), and the average reservoir pressure (P̄). Due to the complexity of the shale reservoir (its typically very low permeability and the presence of a dual porosity system), valid and accurate results of pressure build-up analysis are important to the optimization of individual well completions or depletion plans for gas reservoirs in the shale. Horizontal wells may allow operators to take better advantage of the shale fracture systems and anisotropic flow regime, but they present a new challenge for well testing and analysis.
This paper documents a technical procedure with field examples, using pressure build-up data from horizontal and vertical wells, to assist the reservoir engineer in evaluating the reservoir prior to any decision-making process. This procedure implements two conventional build-up analysis techniques: (1) type curve matching, and (b) Horner's technique. Pressure and pressure-derivative values are used to estimate values of skin, flow capacity, and average reservoir pressure. A newly-developed technique known as the rectangular hyperbolic method (RHM) is implemented in the pressure build-up analysis for comparison to results determined by the previous techniques. The RHM technique is accurate/valid for estimating the various reservoir properties and, in particular, the average reservoir pressure. In addition, reservoir engineering simulation is used to verify the results of the various techniques by using either the pressure build-up data or the production history for the history matching process.