Abstract

The completion technique of fracturing stimulation treatment followed by gravel pack installation, called frac and pack completion, is relatively new to high permeability formations in Bohai Bay Field. This paper focuses on the simulation and evaluation of this completion treatment in Bohai Bay Field. The effects of oil-water contacts, oil-gas contacts, permeability, thickness and mechanical properties of formation on three-dimensional fracture geometry, well skin factor and well productivity have been investigated. The range and degree of these parameters are varied in the sensitivity analysis. A case study from QK17-2 Oil Field addresses the effects of these important parameters on the fracture geometry and well response.

The results of simulation and evaluation show this completion technique is available for many wells in Bohai Bay Field and the application of this completion technique has also some limits. The application of this completion technique to the gas cap reservoir gives rise to the connection of oil and gas through the upward fracture, the rapid increase of gas oil ratio, the drastic decrease of reservoir pressure and recoverable oil reserves. The application of this completion technique to the reservoir with bottom aquifer results in coning through fracture channels and sand production. The paper presents the basic principles of application of this completion to Bohai Bay Field.

Introduction

Frac and pack, a combination of perforating the pay zone, followed by a small-scale fracturing treatment using a viscous fluid, and then gravel packing, is emerging as an important completion and stimulation treatment in moderate and high permeability formations. This completion technique yields a relatively short, highly conductive fracture, which breaches near-wellbore damage, reduces pressure drawdown, decreases near-well-bore flow velocity and stresses, and increases the effective wellbore radius. So it gives rise to relatively high production rate, low skin factor and adequate sand control. This stimulation technique greatly extends the traditional permeability ranges of fracture candidates and has proven to be successful in well completions in the Gulf of Mexico1, Prudhoe Bay2, North Sea3 and other Fields4. The technique is particularly attractive when other forms of stimulation are rendered ineffectively because of mechanical and chemical properties of formation.

Compared with hydraulic fracturing in low permeability reservoirs, the followings are the main stimulation mechanism of frac and pack technique in moderate and high permeability formations:

  1. In moderate to high permeability reservoir, there are higher conductivity and lower flow resistance; thus propped fracture length is less important than fracture conductivity for enhancing productivity.

  2. Pumping pressure is mainly used to balloon fracture width that is helpful to ball out damage zone, decrease skin factor and improve fracture conductivity.

  3. The relatively shorter and wider propped fracture yields effects of increasing effective wellbore radius, lowering pressure differential and fluid flow velocity near wellbore, controlling sand production, gaining higher oil production with lower production pressure differential.

According to the practical experience of employing frac and pack technique in moderate to high permeability formations, the following candidate wells or reservoirs are available:

  1. Wells with deep damage-zone and with high cost for removing formation damage by matrix acidizing treatment.

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