A Systematic Methodology for Extrapolating Gas-Oil Ratio During Declining Oil Production

Abstract

With increasing drive to account for associated gas, it has become necessary to have a reliable technique for extrapolating gas-oil ratio (GOR), commonly used to quantify gas produced with the oil. While there have been significant improvements in the number and quality of techniques available for analysing oil-rate data, GOR prediction has not advanced appreciably. In this paper, a systematic methodology developed on the basis of a new internally consistent GOR model, is presented. Applicability is demonstrated with a field example, which also illustrates its use for estimating recoverable volume of associated gas during oil decline. The good agreement of the results with field data affirms the accuracy of this methodology. While emphasising the importance of consistent data, it is concluded that reservoir and fluid mechanics, as well as operational procedures, are other important considerations when extrapolating GOR. Practical application of this work includes surveillance and management of oil well, reservoir and field. Introduction Several mature oil fields (reservoirs) perform below expectation because of excessive gas production which, in the absence of a gas market, creates serious environmental concerns, sub-optimal performance of surface facilities, and increased operating cost, leaving large oil reserves unproduced. Managing these requires reliable forecasts of the associated gas (AG). In practice, AG is estimated from the producing gas-oil ratio (GOR) and oil production. Traditionally, industry's interest in GOR was limited to its use as a well/reservoir performance indicator. However, with increasing concerns for gas monetisation and environmental performance, the industry is being challenged to improve GOR modelling. One of such challenges is posed by the United Nations' Clean Development Mechanism (CDM) under the Kyoto Protocol(1). The CDM rewards improved environmental performance. For projects recovering and utilising AG that would otherwise be flared/vented, reliable AG (GOR) forecast is a key input for assessment of environmental additionality. The analysis of performance trends (decline curves) of oil and gas production data is widely used for modelling and forecasting. Although traditional techniques implicitly assume constant flowing bottomhole pressure (FBHP)(2–6), with continuous drop of production rate with time, there are modern techniques that correct for FBHP dynamics(7–8). The ideal approach is that which respects reservoir, fluid and wellbore dynamics in predicting production performance.

A Didactic Analysis of Water Cut Trend During Exponential Oil-Decline

“…Adopting the approach used by Lawal et al (9) for gasoil ratio, while oil production declines in a steady-state system; gross rate could increase, decline exactly like oil, decline unlike oil, or remain constant. This underscores the difficulty of reliably describing all possible profiles with a single mathematical equation.…”