Abstract

Dynamic fluid loss through low permeability formations with natural fractures was experimentally investigated in a high-pressure, high-temperature dynamic fluid-loss cell to understand the effectiveness of particulate fluid-loss control additives. The study focused on understanding the relationship between the size distribution and shape of the fluid loss control material on leakoff control and on the cleanup potential.

Sandstone cores of 1-in. diameter and 3-in. length and of approximately 0.05 md were cracked on a press to simulate the naturally fractured low permeability formation. The effective permeability of the fractured cores varied from approximately 0.05 to 2 md. The effective permeabilities of the fractured cores were measured in situ before and after the leakoff test to quantify damage caused by the external and internal filter cake formation. All tests were conducted at 150 F with linear and borate- or zirconium-crosslinked guar solutions and gels. The wall shear rate across the face of the core was varied by changing the flow rate to simulate a typical fracture shear history at the leakoff face. The core and cell were left overnight at the confining pressure (1500 psi) and temperature before flowing back with brine.

It was found that low-permeability formations with natural fractures have characteristics very similar to those of high-permeability formations. Crosslinked fluids not only are good in reducing fluid leakoff, but also provide higher matrix retained permeability than those obtained with linear polymer solutions. The concept of leakoff control effectiveness, which depends on the leakoff control characteristics and the final cleanup potential, is introduced to quantify the depth of invasion of the polymer (internal filter cake) in the core. The leakoff control effectiveness was used as a tool to compare the relative performances of various fluid-loss additives. The results show that the shape as well as the size distribution of the fluid-loss additive can have a significant impact not only on leakoff control but also on cleanup potential.

Introduction

Fluid loss can be described as leakage of the fracturing fluid out of the main fracture. The rate of fluid leakoff (fluid loss) to the formation is one of the critical factors involved in treatment design and determining the fracture geometry. Only the fluid that remains in the main fracture is useful in propagating the fracture, with fluid that leaks off being wasted and potentially damaging. The volume of fluid loss during a hydraulic fracturing treatment determines the fracturing fluid efficiency (ratio of fracture volume to pumped fluid volume). Higher than expected fluid loss can cause early termination of a treatment due to a proppant screenout. The rate of fluid loss also influences fracture closure time, and may influence proppant distribution in the fracture.

Fluid loss to the formation is a filtration process that is controlled by the fracturing fluid composition, flow rate and pressure, reservoir properties (permeability, porosity, pressure, and fluid saturation), and the presence of microfissures, macrofissures, or faults. The efficiency of a fracturing fluid can be greatly improved by adding materials to control fluid loss. These materials, which are often referred to as fluid-loss control additives (FLCAs), synergically form a filter cake with the fracturing fluid on the formation wall. Generally speaking, filtration (and hence fluid-loss control) is a physical mechanism and is dependent on the size distribution of the FLCA.

In hydraulically fractured formations, three regions provide the pressure differentials to control leakoff: filter-cake, filtrate-invaded, and reservoir-fluid zones. The depth of polymer invasion is limited to near the fracture surface, and a dominant filter cake forms quickly. In high-permeability formations, a fourth region exists where polymer and particulates invade, providing reduced effective permeability and additional pressure loss in the particulate-invaded zone. P. 741^

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