Analysis of production data from tight and shale reservoirs requires the use of complex models for which the inputs are rarely known. The same objectives can also be achieved by knowing only the overall (bulk) characteristics of the reservoir, with no need for all the detailed rarely known inputs. In this work, we introduce the concept of continuous succession of pseudo-steady states (SPSS) as a method to perform the analysis of production data. It requires very little input data yet is based on rigorous engineering concepts which works during the transient as well as the boundary dominated flow periods.
This method consists of a combination of three simple and well-known equations: material balance, distance of investigation and boundary dominated flow. It is a form of a capacitance-resistance methodology (CRM) in which the material balance equation over the investigated region represents the capacitance, and the boundary dominated flow equation represents the resistance. The flow regime in the region of investigation (whose areal extent varies with time during transient flow) is assumed to be pseudo-steady state. This region is depleted at a rate controlled by the material balance equation.
The initial flow rate and flowing pressure are used to define the resistance, and the distance of investigation defines the capacitance. The capacitance and resistance are then used in a stepwise procedure to calculate the depletion and the new rates or flowing pressures. The method was tested, for linear flow geometry, against analytical solutions for liquids and numerical simulations for gas reservoirs, exhibiting both transient and boundary dominated flow. Excellent agreement was obtained, thus corroborating the validity of the method developed in this paper.
The proposed method is easy to implement in a spreadsheet application. It indicates that complex systems with complicated mathematical (e.g. Laplace space) solutions can be represented adequately using simple concepts. The approach offers a new insight into production analysis of tight and shale reservoirs, using familiar and easy-to-understand reservoir engineering principles.