The migration of fines into gravelpacks from both unconsolidated reservoirs and through the injection of fluids from the wellbore, is characterised by permeability reduction which may be gradual or instantaneous depending on the invasion process and the pore blocking mechanism. In an attempt to define the maximum allowable levels of fines that can be tolerated by a typical gravelpack which will ensure optimum production capacity over the economic life of the well, an experimental project on pore blocking mechanisms in gravelpacks was initiated in the Department of Petroleum Engineering at Heriot-Watt University in order to define the gravelpack permeability decline as a function of production time.
In this paper, the results of the experimental studies which show the trends of gravelpack permeability reduction as a function of the pore blocking phenomena are presented. Five prevailing pore blocking mechanisms were identified, each characterised by the profile of permeability reduction against time. Based on the characteristic profiles, specific pore blocking mechanism models were developed to quantify gravelpack impairment as a function of gravel-fines size ratio, fines concentration, and fluid production rate. These models have been incorporated into a computerised well performance model which can be utilised to predict both the initial and lifetime performance of a gravelpacked well.
Specific case studies, using field data, to illustrate how the well performance model can be utilised to conduct sensitivity analysis of the productivity index to fines concentration, gravel size and production time. Application of the studies to the analysis of gravelpacked well performance especially with respect to maximum allowable level of fines injection during water flooding for injection wells or formation fines production level for specific commercial gravels are presented. The paper also illustrates how the system can be used to predict possible plugging time of a typical gravelpack.
Gravelpacks are installed primarily to control the migration of load-bearing formation sand grains and other fines from unconsolidated reservoirs. The design of the gravelpack is therefore geared primarily towards:
prevention of sand migration into the wellbore maximisation of productivity through the use of specially processed pack sands (gravel) minimisation of impairment as a result of particle plugging
Achievement of these objectives require the use of rigorously defined criteria in the choice of gravel size. Most gravelpack designs have been geared towards achieving optimum control of sand migration or absolute stoppage with some sacrifice on productivity.