Abstract

This paper discusses processes that are common and un-common in stimulating wells which exhibit medium permeability/multilayered environments. These zones are typically found in the Texas and Oklahoma Panhandles, Southwest Kansas and Western Oklahoma. This paper ranks these processes in order of their cost effective ability to recover additional hydrocarbons from these reservoirs.

In evaluating these methods, a theory will be presented which outlines the boundary mechanism in these areas. This dimension totally controls the effectiveness of the frac treatment. A method of controlling the resultant frac height in this type of environment will be offered.

Value is placed upon processes such as: placing additional proppant in the zone, intense quality control, use of a leading-edge three dimensional real-time frac models, stress logs, calibration tests, reservoir evaluation prior to treatment and perforating. A field example will be cited where these processes were directly beneficial to the net present value of the well.

Introduction

In the history of well completions, the area of stimulating medium permeability formations or reservoirs has been the most often overlooked. By the term "medium permeability" the authors wish to specify the permeability range of 1 to 10 mDs. This range is not tight and not "loose" by most standards. In this range, the FCD starts to work against the stimulation engineer in terms of providing contrast in the pay section. This relationship:

(1)

states that the induced permeability of a proppant pack must be greater per unit of distance from the wellbore If this value does not increase proportionately, then placing proppant farther from the wellbore can become a non-economical option. In other words, there is an equilibrium point where "Bigger is not Better". Industry targets for this ratio are taken from Cinco's work and supplemented to fit the original Agarwal equation so that the target ratio is 31.4159. From manipulation of the relationship, one can easily see that with permeability below 1, the target ratio can easily be reached. At a permeability value of 1, the denominator quits being a "multiplier" and starts dividing into induced perm (conductivity). Since conductivity is a function of the proppant perm multiplied by the propped width, it can be increased by placing more proppant in the induced area, without making that area any longer. Conductivity contrast is lessened by placing additional proppant in excess area, however.

The definition of multi-layered is self explanatory in that the productive zone is not continuous, but separated by "interior" shales or shaley pay sections, that do not produce hydrocarbons.

In the Texas and Oklahoma Panhandle, Western Oklahoma and Southwest Kansas, as well as other areas, many formations exist that fit these descriptions. The successes rate in effectively stimulating them is regional, at best. However, from this should not be the case. It is to the operator's benefit that he have a well of this type, because the stimulation success is considerably easier in these formations due to the multiple layering providing permeability traps, thus bounding or, "framing" the frac treatments to generate a more elliptical shape than in unbounded formations. Stress in the interior shales is not sufficient to provide this effect. Permeability out of the perforated field becomes the more efficient bounding mechanism.

There are several steps that an operator can take that directly effect the outcome of stimulation treatments in these formations. Many recent papers offer insights into building a working theory to optimize stimulation treatments in low perm formations that are bound principally by stress alone. These works offer insight in ways to place proppant more strategically in the pay section and or in the pay section and not below. In those cases, proppant convection appears to be the worst enemy that the stimulation engineer can face.

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