SPECIAL CORE ANALYSIS DESIGNED TO MINIMIZE …

1 CIM 95-136. SPECIAL CORE ANALYSIS DESIGNED TO. MINIMIZE FORMATION DAMAGE associated with . vertical / horizontal drilling applications . By Doane, , Bennion, , Thomas, , Bietz, R, and Bennion, Hycal Energy Research Laboratories Ltd. Abstract well drilling programs. This paper highlights test procedures used in the past, describes some potential Laboratory testing of core material to attempt to problems with these test procedures, and outlines the optimize drilling fluid composition and procedures has current state of the art in laboratory technology with been used for many years to attempt to MINIMIZE invasive respect to specialized laboratory testing to evaluate drilling formation damage of a mechanical.

2 Chemical or biological induced damage for vertical and horizontal well nature which can occur during the drilling of horizontal or applications . vertical wells. This paper discusses the deficiencies of past methods, such as the use of non-representative core CommonMechanisms of Formation Damage During or fluids, non-preserved or non-restored state core, Overbalancedand UnderbalancedDrilling Operations ambient conditions of temperature and overburden pressure, direct injection of muds/filtrates into samples and A number of authors have provided a detailed unrealistically high drawdown gradients for cleanup.

3 The discussion of potential formation damage mechanisms paper describes the current state-of-the-art technology which may occur during overbalanced and underbalanced used to eliminate many of these concerns and also to drilling operations. A summary of this work is provided in extend drilling fluid evaluation technology to extremely the Iiterature(1-6).These mechanisms would include: heterogeneous carbonate and sandstone formations, fractured formations and specific test equipment and 1. MechanicallyInducedFormation Damage procedures used to evaluate the effectiveness and utility a.

4 Physical migration of in-situ fines and mobile of underbalanced drilling programs. particulates b. The introduction of extraneous solids of either an Introduction artificial nature (ie. weighting agents, fluid loss agents, or artificial bridging agents) or naturally Formation damage occurring during the drilling of occurring drill solids generated by the milling action horizontal or vertical wells can be a significant mechanism of the drill bit on the formation. of ultimate reduced productivity in both oil and gas bearing c. Relative permeability effects associated with the formations.

5 In some cases a combination of petrographic entrainment of extraneous aqueous or hydrocarbon and SPECIAL core ANALYSIS techniques are used to evaluate phases within the porous medium. the potential effectivenessof proposeddrilling fluids and d. Formation damage effects associated with the use procedures, prior to the actual cost and risk of of extreme underbalance or overbalance pressures implementation. These tests are conducted to obtain a and associated fines migration or spontaneous better assessment of the risk associated with the use of imbibition phenomena. proposed drilling fluids and to optimize the fluid and e.

6 Direct mechanical glazing phenomena associated procedures which will be utilized in a given horizontal or with bit-formation interactions. This particular vertical well operation to maximize ultimate productivity of damage mechanism is usually associated with gas oil or gas. Considerable advances have been made in drill operations where high bit-rock temperatures recent years in both the executionand interpretation of the commonly occur. results of laboratory coreflow tests to obtain representative results for effective field design of horizontal and vertical 2. ChemicallyInduced Formation Damage.

7 Early Core Flow Test Designs a. Clay induced formation damage associated with the reaction of low salinity or fresh invaded fluid filtrates Figure 1 provides a schematic illustration of an early with potentially reactive clays (swelling clays or coreflow test apparatus utilized to conduct a variety of mixed layer clays). Low salinity or pH shocks may different types of formation damage studies. Many of the also result in clay deflocculatlon phenomena which early studies concerned with drilling induced formation are a disruption of electrostatic forces which are damage focused primarily on damage associated with the holding clays in a flocculated state.

8 This invasion of filtrate from the mud into the formation. Hence, phenomena is common in some kaolinite rich many of these tests were conducted in a si~ljfied fashion reservoirs. using extracted core samples into which fresh water or b. The precipitation of waxes, solids, asphaltenes or proposed mud filtrates were displaced and on which diamondoids caused by a reduction in temperature evaluation of permeability reductions were based. or pressure associated with the drilling process, or However, as described in the previous section detailing incompatibility between introduced hydrocarbon damage mechanisms, potential reactivity with clays or fluids and in-situ hydrocarbon fluids resulting in a in-situ reactivity between invaded fluids and in-situ fluids destabilization and precipitation of asphaltenes.

9 Represents only a small fraction of the potential c. The formation of insoluble precipitates caused by mechanisms of damage which could be induced during a the blending of incompatible drilling and completion typical drilling operation. filtrates with in-situ foreign waters. d. The generation of high viscosity stable water in oil Later test procedures attempted to mirror some of the emulsions in the near wellbore region caused by more realistic effects associated with the drilling process the invasion of incompatible water-based filtrates by utilizing either whole drilling muds or synthetic muds resulting in the formation of an emulsion block.

10 Containing synthetic solids. Many of these tests were e. Wettability alterations associated with the use of conducted in an erroneous fashion by directly displacing invert drilling muds or other muds containing high these muds at an elevated pressure into the samples in a concentrations of polar surfactants or materials. forced displacement or squeeze type mode. Since in a Near wellbore wettability alterations can reduce the natural operation in the field the circulating fluid flows in relative permeability of oil significantly and increase the annular space and a natural filter cake is built up relative permeability to water, causing a dramatic which impedes the infiltration of both solids and mud change in the water-oil production characteristics of filtrate into the formation, this does not represent an a given completion.