In recent times, the oil industry has shown increasing awareness towards the maintenance of optimum well productivity through better drilling/completion practices. Attempts are being made to control the invasion of high permeability sands by mud solids through the use of sized Loss Control Materials (LCM) which can be difficult to clean up. Recent developments involve the use of sized carbonate and sized salt systems with mixed results. Whilst these special systems have been known to provide efficient fluid loss control, the cleanup efficiency especially in horizontal wells has been known to be poor in certain instances. Likewise, the completion of long horizontal sections through highly permeable, unconsolidated reservoirs has witnessed increasing use of prepacked screens many of which are reported to be plugged/damaged by filter cake debris and completion fluid solids as well as formation fines. Gravelpacks are also known to have been damaged not only by formation fines but also by solids in completion fluids injected from the wellbore especially during cleanup operations. The migration of sized particles in the drilling and completion fluids through high permeability reservoir sands, prepacks or gravelpacks is characterised by flow capacity decline which may be instantaneous or gradual depending on the migration process and the pore bridging phenomena. Therefore, accurate prediction of the prevailing pore blocking mechanism for a given pore throat-particle size relationship can provide a good basis for the estimation of the maximum allowable size of particles in various completion fluid systems and also provide a good guide to the design for optimum prepack and gravelpack performance through the use of properly sized gravels. In this paper, attempts have been made to analyse the impact of a number of key parameters on productivity impairment. The analysis has been based on the results of in-depth research into particle-pore bridging phenomena. Based on rigorous experimental studies to define the phenomena, a number of deterministic models have been developed to mathematically define the unconsolidated pay sand/pack sand systems permeability decline profile as a function of invasion pattern, migration/pore blocking mechanism, production/injection rate, production time and fines concentration and fines textural properties. Application of the results to the optimisation of drilling/completion fluids design as well as prepack/gravelpack design and analysis are illustrated with specific case studies. Introduction The oil industry has continued to use sized particulates in many facets of its well completion programmes either as dispersed solids in drilling/completion fluids or as pack sands to control the migration of fines from unconsolidated reservoirs. In all of these cases, particulate size distribution appears to be the only current criterion adopted as a basis for the design of the following: P. 355

To gain a better understanding the difference on microbial community structure for low permeability reservoirs in Daqing Oil Field at different days in outdoor, we constructed the 16SrDNA gene clone library for different days' samples respectively. The results showed that the dominant microbes of zero days' sample are uncultured Acinetobacter sp. (42%), uncultured Clostridia bacterium(21%) and Bacillus sp. (12%); the dominant microbes of two days' sample are uncultured Acinetobacter sp. (53%), uncultured Clostridia bacterium (13%) and Bacillus sp.(10%); the dominant microbes of four days' sample are uncultured Acinetobacter sp. (62%), uncultured Clostridia bacterium (10%), Bacillus sp.(9%) and uncultured Klebsiella sp.(8%); the dominant microbes of 6 days' sample are uncultured Acinetobacter sp. (70%) and uncultured Klebsiella sp.(10%). The numbers of uncultured Acinetobacter sp. and uncultured Klebsiella sp. are gradually increased, the numbers of uncultured Clostridia bacterium, Bacillus sp., Ochrobactrum sp. and Pseudomonas sp. are gradually decreased. It is supposed to provide a dependable basis for the importance for analysing microbial community structure of oil reservoir without delay.

Efficient sand control is dependent on the design of the exclusion system and must take into account a number of formation characteristics and production factors which will affect the eventual well performance. For gravelpacking, the design requires the specification of the size of the commercial gravel to use with the final objective being the achievement of maximum productivity over the production life of the well with effective sand exclusion. To date, a number of formulae have been suggested for gravel size selection, the most popular being the Saucier formula. However in a large number of gravelpacks, operators have reported fines being produced in large quantities, suggesting ineffective bridging by the gravelpack. A review of the available gravel sizing formulae show that they may be too general and possibly too simplistic to take full cognisance of the depositional environment of the formation sand, the operational conditions to which the gravel pack is subjected, the reservoir fluid, the gravelpack stricture, etc. A project has been initiated in the Department of Petroleum Engineering at Heriot-Watt University to address the problem of determining the optimum gravel size under a range of different operational conditions. The project has the objective of developing a theoretical approach and modelling technique to predict gravel size and also to evaluate particle-pore bridging phenomena, using a series of figorously defined experiments. This paper presents the results of the comprehensive experimental investigations carried out to evaluate the bridging effectiveness of various commercial gravels as a function of different operational conditions including the sand sorting and shape. The experimental studies were carried out using a purposely-built packed column to evaluate the effects of production rate, fluid type, formation sand and gravel characteristics, absolute and differential pressure on the efficiency of the exclusion treatment. The bridging efficiency of selected gravels were measured. The experimental data generated formed the data base for developing a semi-empirical bridging efficiency equation which assists in the selection of gravel and the prediction of its bridging effectiveness under specified operational conditions. It also allows the design engineer to conduct a sensitivity analysis of the effectiveness of the chosen gravel to a range of variable conditions. The paper concludes with a description of the computer package being developed for optimum gravel size selection. This package is expected not only to select the optimum gravel size but also predict the eventual well performance. Introduction Gravelpacking has continued to be a popular method of controlling the migration of load-bearing formation fines from unconsolidated reservoirs. However, evaluation of well performance for most gravelpacked wells over the years has revealed evidence of fines being produced in large quantities especially from highly unconsolidated sands, which suggest ineffective bridging of the gravelpack. There is also the evidence of high positive skin factors which are a clear indication of serious formation damage possibly as a result of either one or a combination of the placement technique, inadequate gravel sizing and consequent pack invasion, problems associated with completion fluid used, etc. P. 361^

In this paper, the basic concepts of a 3-D random gravel packing modelling technique being developed to simulate typical gravelpack structures are presented. This is a followup on the 2-D modelling technique which was initiated in the Department of Petroleum Engineering at Heriot-Watt University as part of an on-going project on optimum gravel sizing for effective sand control.

In this paper the details are presented of a software package which represents a new strategic approach to the selection of an optimum gravel size for effective sand control. The new approach not only considers the formation sand size distribution in the choice of the gravel to use, it goes further to evaluate the response of the probable gravels to operating conditions and the overall effect on the well production capacity This software package which operates in a DOS 'pseudo' Window environment is developed on the premise that the best gravel size selection is a function of not only the formaton sand size distribution but also other characteristics such as sand sorting and shape, gravelpack structure which dictates the pack pore throat distribution, and operating conditions and the overall effect on the well production capacity.The design and evaluation tool is made up of:(a). A gravelpack structure simulator (package 1) whose primary function is to carry out a 2-0/3-0 simulation of the gravelpack structure from which the maximum pore throat distribution and corresponding poroperm data and pack structure coefficient are computed. (b). The gravel sizing package(package 2) which is the main subject of this paper is made up of three distinct but interrelated modules namely, the pack structure module, the bridging efficiency module and the well performance module. The possible gravels to use are selected with the pack structure module which permits history matching of the formation sand size distribution with the corresponding pore throat distribution of select gravels. The bridging efficiency module thereafter evaluates the bridging effectiveness of the selected gravels andReferences and illustrations at the end of paper 37 the probable gravel is finally selected on the basis of the overall well performance evaluation.Specific case studies have been considered to demonstrate the use of the package.