Rate Transient Analysis and Investigation of Unit Slopes in Multistage Hydraulically Fractured Tight Carbonate Reservoirs

Abstract

Abstract Carbonate reservoirs constitute about 30% of the world’s most active reservoirs and are thus important. The focus of this paper is on the tight carbonate gas reservoirs that are only economic when produced by completing with a multistage hydraulically fractured horizontal well. A unit slope has been observed on a log-log plot of production gas rate against time. This unit slope and flow regime occurs after the one-half slope indicative of linear flow. The objective of this paper is to investigate these flow regimes and provide a method of analysis. Numerical simulation is the primary method of investigation employed in this paper. Several runs were made with the following variables: reservoir boundary dimensions, hydraulic fracture spacing, phases, hydraulic fracture length and number of stages. It was concluded that the appropriate reservoir simulation model which results in the observed flow regimes in actual well data is a bounded rectangular reservoir containing a centrally located horizontal well with transverse fractures contacting the reservoir boundary. The main flow regimes observed are linear flow and Unit Slope Flow Regime (USFR). Linear flow occurs with drainage from the reservoir matrix into the transverse fractures. The USFR begins when there is interference between the hydraulic fractures and ends when the fractures drain the reservoir as a unit. The appearance of these two flow regimes, either singly or with the linear flow preceding the USFR, is controlled by a critical fracture spacing. An equation is derived for analysis of these flow regimes to obtain useful reservoir properties for characterization.