Dynamic fluid loss tests provide the best method for simulating the fluid loss process during hydraulic fracturing. Through the use of these tests and a reasonable theoretical model for the rate of fracture growth, fracture length can be predicted with acceptable accuracy.

Introduction

Successful design of hydraulic fracture treatments depends upon accurate knowledge of the fluid loss properties of the fracturing fluid. Howard and Fast properties of the fracturing fluid. Howard and Fast gave the first description of the fluid loss process and developed equations relating fracture area to fluid and formation properties and treating data. This development was based upon static filtration tests similar to the API mud filtration test. More recently, Hail and Dollarhide have shown that the static fluid loss test does not adequately represent conditions under which additives perform in a fracture treatment. They suggest using a dynamic fluid loss test to determine fluid loss parameters, and they develop an alternative form of the Howard and Fast equation for fracture area. In addition, they give data demonstrating some of the effects found in dynamic tests that are not present in static tests."Dynamic fluid loss" in this paper refers to fluid leakoff from a fracture when a high flow velocity along the fracture exists at the point where fluid leak-off occurs. This is the normal situation since hydraulic fracturing produces a long, narrow crack along which fluid flows at velocities up to several hundred feet per minute. High velocity is maintained far down the fracture even though volumetric flow rate decreases as the fracture becomes progressively more narrow. When additives are used, laboratory fluid loss data based on the older "static" test (no flow across the surface at which fluid leakoff occurs) are not representative and a "dynamic" fluid loss test should be run. In a dynamic fluid loss test, a high-velocity stream of fluid, which inhibits thick filter cake formation, moves past the rock surface at the same time fluid enters the core. As shown in Figs. 1 and 2 fluid loss during this test can be divided into three regimes:

  1. control by reservoir properties,

  2. control by reservoir properties and filter cake, and

  3. control by steady-state filter cake.

Initially, pressure drop is totally dissipated across the core and fluid leakoff occurs as if no additive were present. Next, a filter cake begins to form and leakoff present. Next, a filter cake begins to form and leakoff velocity is lower because some pressure drop occurs across the cake. Finally, a steady state is reached at which the cake has a constant thickness and fluid leakoff velocity is constant. Flow velocity through the steady-state cake is dependent upon flow velocity across the rock surface, upon fluid and additive properties, and upon rock pore size. In this paper the properties, and upon rock pore size. In this paper the volume of fluid loss required to allow formation of a steady-state filter cake is termed the -spurt lose' (denoted Vsp), and the steady-state cake is described by the leakoff velocity, V(L). Filter cake thickness formed in a dynamic test is limited since particles outside the matrix are subjected to large shearing stresses. To illustrate the type of cake formed, Fig. 3A shows the face of a core exposed to a granular additive in oil and Fig. 3B shows one exposed to gelled water-silica flour (in these tests, additive contacted only a 0.75-in. strip across the core). These photographs show that cake formed by Use additive used in oil is almost entirely within the rock, since all rock characteristics observable outside the test zone are visible inside the test zone.

JPT

P. 882

This content is only available via PDF.