New Results on Decline Curves Considering Non-Darcy Flow Effects

Abstract

Summary Most theoretical work on decline curves considering nonlaminar flow is related to dry gas systems. The purpose of this study is to analyze decline curves considering non-Darcy flow effects in the reservoir for both slightly compressible liquid flow and solution gas-drive systems, keeping the wellbore pressure constant. The transient and boundary-dominated flow periods are examined by means of a finite difference black-oil reservoir simulator with a variable bubblepoint formulation. The consequences of this formulation under non-Darcy flow are also investigated. The influence of the mechanical skin factor on the rate response is also documented. For the case of liquid flow, presented for the first time, it is an expression to evaluate the total skin factor, which is a function of the mechanical skin and skin caused by inertial forces. This expression is given in terms of physical properties of the system, and it is proportional to the square root of the pressure drop. It also equals the square root of the Reynolds number. This is one of the main findings of this work. The possible presence of a semilogarithmic straight line of reciprocal rate for both liquid and solution gas-drive systems is investigated, and new insights are provided. In general, when inertial effects are important and the bottomhole wellbore pressure is kept constant, a true semilogarithmic straight line of the inverse of oil rate does not develop. This result is valid for both liquid and solution gas-drive systems, and it is an important difference in the case of liquid flow under a constant oil rate production, where a semilog straight line of the pressure drop is evident during the transient period considering a large drainage area. For liquid flow, an apparent straight line may be fitted, and the ordinate to the origin is approximately equal to the value of total skin computed with the expression mentioned previously. The presence of inertial effects distorts the shape of the laminar decline curves for both multiphase and slightly compressible liquid flow, causing type curve analysis to yield wrong estimates of the wellbore and reservoir parameters. Also, to perform a history match with data affected by non-Darcy flow by use of a conventional simulator that uses Darcy's law in its formulation, an alteration of the reservoir properties is necessary.