A Study of Induced Fracturing Using an Instrumental Approach

Abstract

Tests to increase knowledge concerning the process of hydraulic fracturinghave been performed on three wells in the Allegheny field in western New York.The fracture orientation and its physical appearance at the wellbore wasdetermined by impression packers run at appropriate times throughout theprogram. Fracture azimuth and its dimensions thus determined were compared tothe rock properties and treatment conditions. Logs to provide data concerningrock properties and to improve identification of vertical fractures were runbefore each step of the fracture treatment. Pressure oscillations were studiedafter each phase of the treatments and surface pressure fluctuations were coinpared to subsurface pressures during treatment. These measurements show thatsubsurface pressures exhibit many of the same characteristics as surfacepressures during treatment of a well; however, transient pressures are dampenedand delayed in time at the bottom of the well when compared with surfacepressures. Data obtained from this study provides a means to help locate anddetermine orientation of vertical fractures made in the open wellbore. Althoughthese tests were performed in only one area, much of the in formation gainedmay be applied to fracturing programs in other areas.

Introduction

In this study, various tests were made to provide data to better understandformation fracturing in situ. The reservoir studied is the Richburg oil sandwhich is a part of a series of predominantly grey sandstones and shales withoccasional brown sandstone layers, known as the Caneadea unit of the Canadawaygroup of Upper Devonian age. Average physical characteristics of 12 cored wellsin this area are 13-ft thick gross sand formation, 12-ft thick net sand, 1.7 mdpermeability, 11 percent porosity, 44 percent oil saturation (before secondaryrecovery) and 16 percent water saturation (before secondary recovery). Thisparticular area of the Allegheny oil field was originally drilled prior to1890. The first waterflood secondary recovery attempt was made in 1929. Thepresent sustained waterflood development began in 1951 and is being carried outon a 4.5 to 5-acre spacing. Objectives of these tests were to (1) studyfracture azimuth and dimension in relation to rock properties and treatingconditions. (2) improve fracture identification techniques, (3) compare surfaceand subsurface pressure during all phases of treatment and (4) compare theperiods of the fluid column oscillations occurring after each phase oftreatment. Although the areas of investigation listed appear divergent inpurpose, the program was designed primarily to increase knowledge of down-holeconditions leading to rock failure and extension of the fracture thuscreated.

TEST PROCEDURES

Wells upon which tests were conducted are designated as Wells 1 through 3.Well 2 was located 440 ft north of Well 1, and Well 3 was located approximately880 ft west of Well 1. Wells 1 and 2 were completed with cable tools, while theproducing interval of Well 3 was rotary cored using fresh water as the drillingfluid to obtain oriented cores. Tests were divided into separate phases on eachwell as the fracturing program progressed and consisted of measurementsperformed prior to treatment, during and after the formation breakdownoperation, during fracture treatment and after treatment. To verify locationand azimuth of the fracture created, and to obtain an impression of wellboreirregularities, a packer was used (Fig. 1). This packer was lowered on tubinginto the wellbore opposite the interval under study and inflated in stages toallow the impression material to conform to the wall of the wellbore. Amagnetic compass was attached to the tool for orientation reference.Impressions made prior to treatment showed no existing fractures in the wellsunder study.