Abstract

The purpose of this paper is to describe the downhole completion equipment presently being used by operators in East Texas to complete wells producing from Cotton Valley formations. Particular emphasis is placed on the factors that influence these operators in placed on the factors that influence these operators in their downhole completion equipment decisions.

Introduction

In this discussion the factors that influence downhole completion equipment selection are broken down into two categories. The first category is composed of the characteristics of the zone being produced that have an effect on proper equipment selection. The techniques being employed to make production from Cotton Valley wells economical vary between operators. The effects of these different techniques make up the second category.

COTTON VALLEY SAND

One area of concern when selecting downhole completion equipment is the environment in which the equipment must perform during the production of well effluents. In the Cotton Valley sand this environment itself does not dictate the use of any specific equipment. For example, pressure differentials placed across the packer during the life of the well are always a primary consideration. With packer depths not exceeding 12,000 feet and completion fluids left in the annulus generally less than 10 lb./gal. in weight, even if the bottom hole pressure were to draw down to zero, the differential pressure placed across the packer from the top would not exceed 6250 psi. Bottom packer from the top would not exceed 6250 psi. Bottom hole pressures experienced during production from the Cotton Valley sand generally do not exceed 5500 psi. These pressures do not rule out the use of almost any production packer available to the petroleum industry. production packer available to the petroleum industry. In addition to pressure differentials, bottom hole temperature and the presence of corrosive agents in well effluents must be taken into consideration when selecting downhole completion equipment. Here again, in Cotton Valley sand completions the temperature at the depth of the packer does not exceed 275 deg. F, and the presence of H2S and similar corrosive agents has not been a problem. Consequently, it is apparent that conditions affecting the packer during well production allow the use of any of the three types of packers available: permanent-drillable, permanent-retrievable, or hook-wall type packers.

A brief description of these packers may be helpful at this time. Permanent-drillable and permanent-retrievable packers fall into a more general permanent-retrievable packers fall into a more general category of permanent type packers. Permanent type packers are generally run independently of the tubing and packers are generally run independently of the tubing and effect a seal against the casing wall. A seal assembly made up on the lower end of the tubing is then run and located in the polished bore of the packer effecting a tubing to packer seal. Hook-wall type packers are run into the well on the end of the tubing string and set by some type of tubing manipulation.

For operators who decide to do some or all of their initial well stimulation through the tubing and under their production packer, special consideration must be given to designing downhole assemblies that will perform satisfactorily in this environment. This is especially true in Cotton Valley sand wells because massive hydraulic fracturing typically is necessary to obtain economic production. Two aspects of this through-tubing stimulation which must be subjected to close scrutiny are:

  1. tubing movement as a result of pressure and temperature changes and

  2. pressure differentials to which the packer might be subjected.

In the event that a ball out or screen out should occur during stimulation jobs which often exceed 100,000 gallons in volume and rates of 15 barrels per minute into a formation with as much as 280 deg. F temperature, considerable tubing contraction can be anticipated. Formulas and computer programs for predicting the effects of pressure, rates, volumes, and predicting the effects of pressure, rates, volumes, and temperature on tubing movement are readily available. A discussion of the calculations involved in these tubing movement predictions is beyond the scope of this paper. It is sufficient to say that estimates of the extent of tubing contraction should be obtained in order to minimize the possibility of subjecting either the tubing or the packer to stresses which are beyond their designed limitations.

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