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Abstract

Fracture height growth can be contained by selective placement of a low mobility bed of proppant along the upper and the lower tips of a propagating hydraulic fracture, while keeping the front tip open to further lateral extension. This is done in a separate treatment step prior to the main treatment. The success of these treatments in reducing height growth depends on the proper design and execution of the treatments, based on knowledge of the vertical stress profile, which dominates the fracture geometry evolution in three dimensional space. Such barrier placements are limited by the rate of early fracture height growth with respect to extension, treatment rate, fluid viscosity, proppant concentration, proppant size and specific gravity. This paper shows the effects of these variables on the effectiveness of the placement of artificial barriers. The paper also describes the three dimensional fracture geometry evolution beyond the lateral extent of the emplaced barriers. Beyond these barriers the fracture loses any artificial containment, and grows according to the natural stress profile. The possibility of interactive diversion through convective settlement is also studied.

Introduction

Fracture height containment is a common challenge faced in the design of hydraulic fractures, particularly in low permeability reservoirs. Development of artificial barriers by placing a low mobility, or bridged, proppant pack along the upper and lower tips of a fracture, often inhibits fracture height growth through these tips. However, beyond these barriers the fracture loses the artificial containment and "mushrooms" in the natural rock stress regime. Mukherjee, et al. present case studies with field examples where such barriers were effectively placed with substantial production improvements due to increased effective fracture half-length. However, a general study of the process of artificial barrier placement and an appropriate set of guidelines for such placement is not reported in the available literature. This paper is intended to fill this gap.

In the placement of artificial barriers, understanding the process of proppant movement in the fracture is very important. Proppant movement is controlled by both individual particle settling or floatation, depending on the particle density, and also the convective movement of the particle slurry. The upper and the lower barrier experience different settlement, or segregation rates during placement of the diverting materials in the fracture channel. A brief discussion of the factors affecting these rates is presented. Also, with proper sensitivity studies of pump rates, proppant densities, slurry concentration, etc., a set of guidelines for the placement of artificial barriers is presented.

Particle Movement During Placement of Artificial Barriers

The development of artificial barriers to fracture height growth relies on placement of banks of sand or other particulates. These banks exhibit a high resistance to movement, and also restrict the transmission of fluid pressure to the fracture tips. Placement of the barriers relies on both single particle settling and convective slurry settling in the fracture. Equations to predict slurry and particle settling have already been presented.

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