C-Curve And Independent Oil-Rate And Oil-Cut Decline

Abstract

Abstract Decline analysis is an important working tool, particularly for reserves evaluation in producing fields. However, its theoretical foundations are not fully understood and much work remains to be done to develop the theory and application of the method. Based on practical experience of fitting decline curves to both field production and reservoir simulation results, the authors have identified two short-comings of conventional decline analysis, together with partial solutions to these problems. It is estimated that the two improvements together will lead to a significant improvement in the accuracy of decline analysis reserves evaluation. The first problem is that of the limitations of hyperbolic curves and other alternatives to exponential decline. When field production or reservoir simulator results show non- exponential decline, the decline rate usually reduces in time (e.g. from 25% per annum to 15%, 10% etc), until levelling out and then continuing at a more-or-less constant rate (e.g. 5% per annum). In contrast, hyperbolic curves show decline rates than keep on reducing, eventually heading to a zero decline rate. In consequence, hyperbolic decline curves can give rise to unrealistically high ultimate recovery values. Instead of hyperbolic curves, an alternative generalisation of exponential decline is presented – the "C" or "cumulative" curve. These curves take a simple form; they include hyperbolic curves as a subset; and they allow the late-time decline rates to approach a non-zero asymptote. The second problem is that there are cases where fitting a decline curve to the oil-cut curve gives a very unrealistic prediction of oil-rate, and vice-versa. A way of addressing the problem that has been found to be very successful is to fit independent decline curves to oil-cut and oil-rate and then use both simultaneously to predict how the gross-liquid production rate varies with time.