Evaluation of Formation Damage Caused by Drilling Fluids, Specifically in Pressure-Reduced Formations
Authors: Marx Claus , Rahman S.S. 
Published: December 2007

Summary
This paper describes a method for evaluating formation damage caused by drilling fluids in reservoirs that may have pressure considerably lower than hydrostatic pressure. The problem is of specific interest for EOR and/or underground gas storage projects. The method, which is flexible and practically oriented, allows formation-damage evaluation under the following conditions: - 1,400-psi [9.7-MPa] differential pressure, 300 degrees F [150 degrees C] temperature, 6-ft/sec [2-m/s] annular velocity, 0.4- to 1-in. [1- to 2.5-cm] core diameter, and 10-in. [25-cm] length.

Introduction
Formation damage is evaluated by means of two factors: damage ratio (DR) and sectional damage ratio (SDR). The residual permeability is expressed in terms of relative values, with the initial permeability is expressed in terms of relative values, with the initial permeability as reference. The depth of permeability impairment is permeability as reference. The depth of permeability impairment is determined by measurement of the permeability of 2-in. 15-cm] -long segmented cores (Fig. 1). For this criterion, the SDR term is introduced. The method described here was applied for evaluating formation damage caused by a KCl/chalk mud in two sandstones of 10 and 1,000 md range with pressure difference, temperature, annular velocity, and time of contamination as the influencing variables. The permeability of the formation at or near the wellbore may be substantially reduced as a result of the contamination by drilling fluids. There is no generally accepted method for evaluating formation damage under borehole conditions. The objective here is to describe a method that allows evaluation of formation damage caused by drilling fluids and to present results obtained from this method in terms of DR and SDR.

Description of the Test Equipment
The test facility developed for formation-drainage evaluation has three main components (see Fig. 1): core holder, circulating system, and backpressure system.
Core Holder. The core holder is constructed according to the principle of a Hassler cell. It can take cores 0.4 to 1 in. [1 to 2.5 cm] principle of a Hassler cell. It can take cores 0.4 to 1 in. [1 to 2.5 cm] in diameter and 2.5 to 10 in. [6 to 25 cm) long. Cores with a total length of 10 in. 125 cm] may be mounted in one piece or segmented in sections (e.g., 2.5 in. 16 cm] long) without any noticeable change to DR or SDR values. Segmented cores can be used to determine the depth of damage. The rubber sleeve may be subjected to a pressure of 2, 1 00 psi [ 14.5 mPa] to avoid any possible bypass. The core holder may be heated to a temperature of 300 degrees F [150 degrees C]. The temperature is measured at the core. Drilling-fluid contamination of the mounted core can be applied under dynamic or static conditions at pressure levels up to 1,400 psi [9.7 MPa] and temperatures up to 300 degrees F [150 degrees C]. psi [9.7 MPa] and temperatures up to 300 degrees F [150 degrees C]. Dynamic conditions simulate the filtration process during the drilling operation. Because of shear stress between the drilling fluid and the mud cake, erosion of the mud cake takes place and causes higher filtration rates. Static conditions prevail when the mud is not flowing, the pressure difference between the hydrostatic pressure of the mud column and the pore pressure leads to filtration, and buildup of filter cake is undisturbed. pressure leads to filtration, and buildup of filter cake is undisturbed. Circulating System. The circulating system is a closed circuit with a pressure reservoir for securing a constant pressure. The pressure level is kept constant by the gas volume in the pressure reservoir. A precision piston pump with a capacity of Pmax =2,100 psi [14.5 MPa] and qmax,=24 gal/hr 10.09 m-3/h] and a variable-speed drive is used to force the drilling fluid through the circuit. The annular velocity can be adjusted to 160 ft/min 10.8 m/s] by positioning of Plate 18. Fig. 2 shows the front part of the core holder and the Plate 18. Fig. 2 shows the front part of the core holder and the fluid path across the core face. A heat exchanger is placed within the pressure reservoir to keep the drilling fluid at the predetermined temperature. Separation of the mud solids in the pressure vessel (Item 6 in Fig. 1) is avoided during circulation by placing the fluid entry at the top of the vessel and the suction line at the bottom. When circulation is interrupted for a longer period of time, the drain valve is opened and the mud container is emptied.
Backpressure System. To secure the desired pressure difference across the core, a pressure behind the core is established and precisely regulated. The filtrate volume and/or rate is measured in a precisely regulated. The filtrate volume and/or rate is measured in a calibrated cylinder that may be subjected to pressure up to 700 psi [4.9 MPa].
Measuring System. Ail important parameters for the evaluation of the test results are carefully measured. These include drilling fluid pressure, pf, backpressure, pb, pump rate of the drilling fluid, qj, temperature of the drilling fluid, Tf, temperature of the core, Tc, and the rate of filtration, qF. The permeability of the cores is determined both at the beginning and at the end of the contamination period. The test equipment was used extensively to check its reliability for formation-damage evaluation.

Test Program
Two sandstones of different permeabilities were used to evaluate the formation damage caused by a KCl/chalk drilling fluid. The type of mud used for the investigation is frequently used in northern Germany. Formulation and physical properties are listed in Table 1. The main influential parameters-differential pressure. temperature, annular velocity, and period of contamination-were varied over a wide range. The details of the test program are presented in Fig.
presented in Fig.
Each test was run with new cores in. [2.54 cm] in diameter and 5 in. 112.7 cm] in length. The petrophysical data are listed in Table 2. The cores, which were drilled with a diamond drill bit, were taken from a large block obtained from a quarry and represent the outcrops of a known oil-bearing stratum. Water was used as the drilling fluid.
JPT
P. 1449