The Use Of Grasp, A Finite Element Program, To Model Faulted Gas Reservoirs Of The Southern North Sea Basin
Authors: Goldwater Mike H. , Collins Paul A. , Taylor Barbara A. 
Published: December 2007

Abstract
This paper describes a single-phase, 2-dimensional reservoir simulation program called GRASP (Gas Reservoir Areal Simulation Program) and its use in modelling gas reservoirs in the southern North Sea.
The design of the program is outlined, placing special emphasis on those features which have proved to be of greatest value during its application to various gas fields. These include the ability to describe pressure discontinuities across faults and overlapping gas bearing strata within the context of a 2-dimensional model. The provision of a network analysis model to describe the surface gathering system allows the program to be used either for history matching or for predicting the future reservoir behaviour. In the latter case, facilities are provided which allow the consequences of different operating strategies to be investigated. GRASP has been designed, not as a research tool, but as a practical aid to reservoir analysis. Most published applications of finite element methods have considered only small regular reservoirs. In the present paper particular attention is directed towards the difficulties encountered in designing a program capable of describing large, realistic reservoirs.
The program has been used to model three of the gas fields in the southern North Sea; Indefatigable, Leman and Viking. The Viking field has been studied with particular reference to the effect of various operating strategies. The paper uses the Indefatigable and Leman studies to provide examples of the results and output provided by GRASP and its output analyser (GROUT).
The success of GRASP has confirmed the considerable advantages of finite element methods over finite difference methods, particularly in the treatment of faults and overlapping strata and in the flexibility with which refinements of the model in regions of special interest may be made. It has been found that the size and execution times for GRASP are usually significantly better than a comparable finite difference program for a given level of accuracy.

1. INTRODUCTION
In the year 1976/77 the British Gas Corporation purchased 39.4 billion cubic meters(1) of gas from the oil and gas fields of the North Sea. Through its subsidiary companies, British Gas is also involved in the exploration and development of hydrocarbon reservoirs both in participation with other companies and on its own.
The Exploration Department of British Gas carries out computer simulation studies of reservoirs in connection with its operating responsibilities and during contract negotiations. Although simulation programs are commercially available, these do not meet the special needs of British Gas. It was therefore decided to develop a simulation program which would meet these requirements and provide valuable 'in-house' expertise. Most fields have special features which make small changes to any program necessary before a realistic model is obtained. If the program user has a continuing working relationship with the developers, modifications may be carried out faster and with greater reliability.