Abstract

Fracture vertical propagation can be motivated by the need to prevent possible water or gas breakthrough from adjacent layers. It can also be inadequate when a contained fracture is required to optimize the area drained by the fracture. The usual problem encountered in the field is to overcome adverse containment conditions. Hence, the need for controlling fracture height growth can be important in particular when fracturing formations with small or nil stress contrast.

This paper presents the successful application of a methodology to control fracture height growth through the selective placement of an artificial barrier above and/or below the pay zone. Five case studies are presented where radioactive tracers in the proppant were used. Pre and post-fracture gamma rays are presented in each case to confirm or not height containment to the zone of interest. The paper discusses the procedure followed in each case. Details of the execution, rates, fluid and diverting material are presented.

In addition, pressure analysis is provided from measured dead string-bottom hole-pressures. Optimization of fracture length and amount of proppant is obtained from control of fracture height growth in each case.

Finally, production optimization of the reservoir is showed by comparing production rates and declines of wells fractured with and without height control.

Introduction

The ability to control vertical propagation of fractures has always been of considerable interest in hydraulic fracture treatment designs. In fact, the production increase in low permeability reservoirs is directly related to the fracture drainage area of the production interval. Containment of the fracture to the productive zone of these low permeability intervals should provide a greater production increase.

The relationship between fracture penetration, fracture conductivity ratio and production increase was provided by Mcguire and Sikora. These curves represent fracture penetration as a fraction of the drainage radius. If a good conductivity ratio can be achieved, then a fracture penetrating 100% of the drainage radius can provide as much as a 13-fold increase in production. When a fracture is initiated in a productive interval, it will grow in a vertical, radial pattern until it encounters resistance to this growth pattern. Such resistance is normally a barrier rock or a rock strata that contains higher stress and normally is tougher and less permeable that the productive rock. If the barrier rocks are too weak to withstand the pressure required to propagate the fracture, then the barrier rocks will rupture and vertical growth will continue. In this case, production increase could be very poor because much of the fracture area could is outside the zone of interest and penetration severely limited.

One other important reason to control fracture height growth is when there is an adjacent water or gas zone. In both cases fracture height growth can result in the production of undesirable fluids. In case of water, this not only increases operating costs in order to dispose the water but will normally limit the oil production of the well. Ultimate recovery could also be severely reduced if water coning occurs or the water cut becomes too high to allow profitable operation. Where a gas cap exist, upward fracture growth could stimulate the gas production at the expense of oil production increase. This could also result in premature depletion of reservoir energy and reduced ultimate recovery. Hence, in some cases optimum lateral extension of the fracture and production optimization cannot be achieved because the fluid, proppant and pumping energy are misused in propagating the fracture out-of-zone. Thus, the interest in the ability to control fracture height growth by artificial means.

Fracture Growth Control

The dominant factor in controlling vertical height growth is the stress contrast between the pay zone and the barriers. P. 75

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