Abstract

This paper presents a gas field development model that combines certain facets of previously published ones. The manner of combination has required the development and use of some new and little-known relationships, permitted accurate future performance predictions, eliminated several of the simplifying assumptions that might introduce gross inaccuracies, and removed the necessity of referring to plotted curves. This model which was designed for microcomputer solution, permits the analytical prediction of a gas field to meet a gas sales contract length and rate, based on reservoir capacity, individual well and equipment capacity, and the state of depletion of the reservoir as predicted by material balance. Based on the model predictions, recommendations concerning drilling and completion schedule of additional infill wells and the need for compression facilities, to meet the gas sales contract, have been made.

Introduction

A gas field future performance prediction model requires iteration on the pressure with a corresponding change or iteration on gas properties and rock properties. Accordingly, it would be very helpful for engineers who work with gas reservoirs and have access to computer to have a computer subroutine for evaluating gas properties from the composition and the other readily available properties normally measured on the fluids, as well as rock properties. If a gas is to be sold, component analysis is always made of that gas. Of the data required to perform an accurate future performance predictions are the dew-point pressure and volume percent liquid formed during retrograde condensation. In the absence of laboratory measurements, analytical expressions can be used to provide that data that may not be less accurate than that measured in the laboratory.

Performance prediction would be optimistic without proper accounting for reduction in permeability caused by increase in net overburden pressure and the loss in productivity due to accumulation of condensate liquid around the well-bore. The degree of error depends on the ratio of the abandonment rock permeability to original rock permeability, volume percent liquid formed during retrograde condensation, and relative permeability characteristics. Unconsolidated sandstones with high clay content show the maximum degree of change For overpressured gas-condensate reservoirs, effective permeability to gas must be treated as a function of pressure and liquid saturation

Surface and well-bore equipment capacities, coupled with a detailed reservoir description, are necessary for an accurate evaluation of production rate forecast. In order to achieve this objective, allocation of the original hydrocarbon in place among producing wells should be made, based on production and pressure history of wells. For wells that have not yet been produced, an estimate of the original hydrocarbon in place should be made. Total field production forecast, should meet the gas sales contract length and rate. In order to satisfy the estimates of demand, a completion schedule of additional infill wells and the need for compression facilities have to be determined. To perform the tedious trial and error calculations, this model was designed for computer solution.

The material balance for a gas reservoir is undoubtedly, the simplest material balance the reservoir engineer encounters. Nevertheless, we must proceed cautiously when we apply the material balance to retrograde condensate reservoirs and/or abnormally pressured gas reservoirs.

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