A Method of Analyzing Performance of Gravel-Pack Completions in Seria Field, Brunei

Abstract

A method of evaluating gravel-pack completions has been developed that allows the use of small samples of observed data with large variance to be checked against theoretically derived values-in this case, the normalized productivity indices. The results give insight into the mechanics of productivity indices. The results give insight into the mechanics of impairment and lend support to previously suspected causes of impairment. Introduction Brunei Shell Petroleum (BSP) Co. Ltd.'s Seria field produces from shallow, unconsolidated, depleted produces from shallow, unconsolidated, depleted sands. The majority of wells require sand control during their production lifetime, and over the years many gravel packs, both openhole and inside packs, have been placed. The productivities achieved by the various gravel-pack repair techniques have been difficult to analyze due to the few reliable data points and wide range of values. Therefore, a statistical test was used to compare actual productivity indices (PI's) before and after gravel packing with theoretically derived PI's. The effects on productivity of different types of gravel packing and different gravel sizes also were compared. packing and different gravel sizes also were compared.To do tills it was necessary to establish theoretical values for the specific PI's (RB/D/psi/ft) of the various pack configurations using Seria reservoir data and completion sizes. The changes in well productivity as a result of gravel packing were calculated theoretically for the various gravel-pack types and placement methods by considering the individual pressure drops through the sequential resistances to flow imposed by gravel and liner and by considering these resistances as apparent skin factors in the flow equation. The equations were developed using typical Seria field reservoir data and completion sizes. Gravel-Pack Types There are two basic types of gravel packs used in BSFs Seria field: openhole gravel packs (OHGP) and inside gravel packs (IGP). Openhole Gravel Packs An OHGP consists of a slotted liner or wire-wrapped screen in an open hole, packed around with gravel. The openhole section normally is underreamed to increase the effective wellbore radius and pack thickness. This is achieved simply in a new completion; in an existing completion this requires milling a window in the casing, sidetracking, underreaming, and then gravel packing. This is referred to as a sidetracked underreamed gravel pack (STUGP). pack (STUGP).OHGP's allow completions with little apparent PI impairment and not too severe PI decline with continuing production. It will be shown later that in practice severe pack impairment does occur but practice severe pack impairment does occur but largely is offset by the increase in effective wellbore radius as a result of the underreaming. JPT P. 496