Abstract

The Michigan Basin is widely viewed as having only mature reservoirs with limited availability for new treatment designs. The latest re-stimulation program using CO2 might give rise to the opportunity of enhanced production in Michigan as well as benefit new completions.

The Prairie Du Chien (a.k.a. PDC) is a sandstone formation with depths that vary from 8,000 to 11,000 feet and formation thicknesses ranging from 50 to 200 feet. Stimulation treatment designs over the past two decades since the PDC's initial drilling have been marked by vast differences in natural gas production numbers from wells within the same field. Restimulation treatments have included cross-linked fluid or nitrogen foam, providing mixed success rates. Problems such as excessive surface treating pressure, limited down-hole sand concentration, early screen outs, and water retention have been experienced in these stimulations. Many existing wells within established PDC fields have declining production numbers despite reasonable reservoir pressure still present.

Formation characteristics and well history will be detailed to shed more light on the current wells and re-stimulations taking place in the PDC. The advantages of the CO2 treatment design marked with increased fluid recovery capabilities by gas assist along with the hydrostatic advantages will be expanded upon. Treatment design details with pressure response along with production results from the first set of wells will be included.

This paper will expand re-stimulation ideas as well as spur a desire for further investigation into the use of CO2 in the Prairie Du Chien. It also hopes to change the perception of the Michigan Basin to one of a viable, rediscovered source of natural gas in the ever tightening world supply.

Introduction

The knowledge gained by four CO2 re-stimulations over the time period from June through November 2006 will be detailed within this paper. The argument will be made for the use of CO2 in the re-stimulation of existing PDC wells in the Michigan Basin as well as the initial completion of newly drilled wells. A brief summary of the PDC formation along with characteristics and rock properties will be presented. Basic information regarding liquid CO2 as well as its use in hydraulic fracturing stimulation treatment will be shown. A look at past stimulation treatments with various re-stimulation techniques will be listed also including their problems and design challenges. The advantages of CO2 will be described and the treatment data of the four wells presented. The data will include fluid summaries as well as proppant information along with the pressure response of treated wells.

A Reciprocal Productivity Index (RPI) analysis will be shown from well data after treatment that included surface pressure, gas, and fluid volumes. This analysis will show an increase in effective fracture length, an increase in pressure connected drainable area, and an increase in the pressure support available at the formation face. Also supporting the value added by the re-stimulating is the improved NPV as well as an increase in estimated ultimate recovery.

Prairie du Chien (PDC) Overview

(For completeness and through-out this paper, when the term PDC is used, also included are the upper, middle, and lower PDC facies as well as the St. Peter sandstone. Of the four wells, some were commingled PDC/St. Peter.)

In terms of a geological assessment, the PDC is considered to be within the Middle Ordovician sandstone gas play. The PDC is one of the deepest gas plays within the Michigan Basin, having depths ranging from below 6,000–8,000 ft to 11,000–12,500 ft in the basin center. The PDC formation in the central basin has a thickness of up to 1,200 ft with actual pay zone thickness in the range of 50 to over 200 ft in the central basin. Along the edges of the basin, the formation and pay zone ranges decrease. Within the pay zones, porosity can be in the range of 5–20 percent with an average around 10 percent. Rock characteristics include sandstone, which is a silica-cemented clean quartz interlaid with shaly dolomites. For further study and a complete analysis of the structure, stratigraphy, lithology, and drilling history, please see the listed reference material at the end of this paper.

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