Influence of Downhole Conditions on the Leakoff Properties of Fracturing Fluids

Abstract

Summary. Fluid loss under dynamic conditions, considering such realistic conditions as low-permeability core samples and extreme shear conditions, has been examined and is reported in this paper. Dynamic data of fluid loss as a function of time, pressure, and temperature have been developed. Introduction The leakoff velocity of complexed fracturing fluids is highly dependent on environmental conditions. Previous work has shown that leakoff increases as the shear rate increases. This work examines the combined effects of shear rate. pressure differential, temperature, and time on the leakoff profile in complexed gets. Result have shown that C under simulated reservoir conditions is not proportional to p and t as proposed by classic theory. C was found to be proportional to an average p, as proposed by Nolte. Leakoff velocity follows a t relationship, where m=0.5 to 1.0, depending on shear rate, pressure differential, and time at shear. An equation is presented whereby the leakoff velocity at each increment of the treatment can be calculated on the basis of changing environmental conditions. Experimental work to investigate fluid loss under dynamic conditions has been previously reported. Many of these attempts did not adequately model conditions existing in a fracture. Low-permeability core samples, unrealistic flow profiles, pressures that were too low, and unrepresentative shear conditions were some of the objections raised as to the validity of the test results. This paper describes work where these factors have been taken into consideration during test design. Laboratory data are presented that describe the effects of shear rate, pressure differential, temperature, and time on the leakoff profiles of complexed gels. The relative effectiveness of fluid-loss additives, such as diesel and silica flour, is also present-ed. A comparison is made to other dynamically obtained leakof present-ed. A comparison is made to other dynamically obtained leakof velocities, as well as static data. Although dynamic data have been available, they have not been applied in field fracture design. In this work, a method of converting dynamic data to an effective C that can be entered into today's fracture simulators is proposed and examples are given. Experimental Procedures Core Preparation. A diamond-bit drill was used to cut 0.94-in. [2.38-cm] -diameter stocks from Ohio, Bandera, and Berea sand-stone quarry blocks. The stocks were dried in a vacuum oven for 12 hours at 250 deg. F [121 deg. C). Epoxy was then applied to the stocks and allowed to harden. A diamond-bit saw was then used to cut 1-in. [2.54-cm] -long core plugs from the stocks. The core plugs were dried in a vacuum oven for 12 hours at 250 deg. F [121 deg. C]. A beaker containing core plugs was placed under a vacuum of 0.19 × 10 psi [0.0013 kPa] for 4 hours. At the end of 4 hours, 2 % KCl was introduced to the evacuated container. The cores were allowed to saturate for a minimum of 10 days. The liquid permeability to 2 % KCl was measured on each core at the end of this permeability to 2 % KCl was measured on each core at the end of this period. period. Fluid-Loss Cell. Fig. 1 shows the fluid-loss cell with a 1 × 2.3 × 0. 13-in. [2.54 × 5.95 × 0.32-cm] slot passing over a 1.75 × 1.75 × 1-in. [4.45 × 4.45 × 2.54-cm] core. The slot width can be varied by interchanging heads. Only a 0.13-in. [0.32-cm] slot was used for these experiments. The auxiliary equipment, such as interconnecting tubing and heat exchanger, was sized to provide the same shear rate as produced in the slot. Fluid-Loss Testing. The single-pass system used to conduct the dynamic fluid-loss tests is illustrated in Fig. 2. The complete system contains a test fluid reservoir, a fluid injection pump, a crosslinker injection pump, a heating bath, a dynamic fluid-loss cell, two ballast chambers, and a backpressure regulator. The test fluid was premixed both with and without fluid-loss additives for a minimum of 30 minutes before each test. When needed a delayed crosslinker was added to the suction side of the dual-piston fluid injection pump by a syringe pump. Pumps were preset and tested at the desired flow rates before the test started. The fluid crosslinked as it flowed through 50 ft [ 15.24 m] of coiled 0.37-in. [0.95-cm] -OD copper tubing heated in a hot oil bath at 185 deg. F [85 deg. C]. Heating tape was used to wrap the fluid-loss test cell, and the temperature was set at 175 deg. F [79 deg. C] with a temperature controller. The temperature of the fluid exiting the fluid-loss cell w measured with an in-line thermocouple attached to an electronic display. Two ballast chambers were installed in the test system to maintain constant flow rate and to eliminate any system pulsing. When particulate fluid-loss additives were used, a knockout pot was added to the test system. The knockout pot was inserted directly in front of the backpressure regulator to prevent plugging by the particulate fluid-loss additives. Filtrates exiting the fluid-loss test particulate fluid-loss additives. Filtrates exiting the fluid-loss test cell were measured at various time intervals by continually weighing a collecting vessel containing the eluant on an electronic balance. Data Analysis As Penny et al. described, dynamic fluid-loss data can be analyzed with a plot of log(volume/area) vs. log(time) where slope, m, is an exponent of time and the y intercept, b, is the leakoff rate (cm/min ). For comparison, an equivalent C at t can be calculated from this plot with the following equation: (1) where Z = antilog(log F, - m log t). Substituting m=0.5 and t=36 minutes into Eq, 1, we obtain C at t =0.0164 antilog(log F -0.5 log 36), which further reduces to C at t =0.0164 antilog(log F -0.78). Picking our F values from the plot of various shear rates and pressure differentials and solving Eq. 1, we obtain C (ft/min) values from our dynamic test data, which are shown in Figs. 3 and 4. SPEPE P. 43

Influence of Shear History at Bottomhole Temperature on Fracturing-Fluid Efficiency

“…The test conditions reported here produce a comparable a slick surface on the core face that inhibits the filter-cake buildup. 13 The leakoff profile for fluid that has been sheared for a prolonged period before the fluid-loss test is run is different from that for fluid that is simply sheared for a few minutes before the test. Crosslinked fluids run with the same cell using' Ii sil1gle-pass flow system show a high degree of shear-rate dependence.…”

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A New Technical Standard Procedure To Measure Stimulation and Gravel-Pack Fluid Leak-Off Under Dynamic Conditions

“…3,4 Laboratory testing of leak-off is still helpful in characterizing optium fluid properties and as a starting point for leak-off modeling. Actual fluid loss values can often differ from laboratory tests.…”