Two stimulation operations have been conducted to date in the paludal zone of the Mesaverde formation in one well, MWX-1, at the DOE's Multiwell Experiment test site in the Piceance Basin near Rifle, Colorado. Problems were encountered in the second stimulation: MWX-1 would not sustain production for several months and post-frac production production for several months and post-frac production was less than pre-frac rates. The laboratory program was expanded to examine these problems and program was expanded to examine these problems and these laboratory studies were integrated with well testing and other data to help explain MWK-1 production behavior. A unique explanation cannot be found production behavior. A unique explanation cannot be found for the failure of MWX-1 to produce; a combination of factors was responsible. Water probably inhibited matrix rock production. A system of naturally occurring microfractures is important in production from the paludal zone and it probably sustained damage by water and fracture fluids. The basic gel degraded slowly because only a small amount of breaker was used. The fracture closure (viscosity break) observed from the volte analysis of the stimulation was not the same as the breakdown of the basic gel. The remedial treatment conducted after the second stimulation was probably too reactive. A list of items has been developed from experience gained both inside and outside the laboratory that shows what work and which procedures should be emphasized or avoided in tight sand stimulations.
Two stimulation operations have been conducted in Well MWX-1 of the Department of Energy's Multiwell Experiment (MWK) in the Piceance Basin near Rifle, Colorado. Zones 3 and 4 of the paludal section of the Mesaverde Formation, shown in Figure 1, were the zones of interest. The first stimulation operation (Phase I, December 1983) consisted of a series of small, unpropped step-rate/flow-back tests and minifracs whose primary goals were fracture diagnostics and containment prediction. The second (Phase II, May 1984) was a larger sandpropped hydraulic fracture treatment with the additional goal of production enhancement.
Production rates of 250 MCFD (3080 m /d) were measured from the two zones together during prePhase I testing. No difficulty was encountered in prePhase I testing. No difficulty was encountered in getting the well to flow after periodic cleanup during the winter after the Phase I operations. However maximum flows of only 200 MCFD (5660 m /d) could be sustained just prior to Phase II operations. Clean-up after the Phase II Phase II operations. Clean-up after the Phase II stimulation was difficult and the well was not capable of sustained flow. A remedial breaker treatment was performed a month later (June 1984) without notable improvement. Sustained gas flow was not realized until mid-July when a revised packer assembly was installed. Nevertheless, rates in excess of 170 MCFD (4810 m /d) could not be sustained during the ensuing post-frac testing period. period. Extensive laboratory investigations were initiated to examine the production problems. These studies involved many aspects of the core program and the analyses which had been performed prior to both stimulation operations.
The purpose of this report is to examine (a) early core analysis data, (b) laboratory core studies supporting stimulation design, and (c) post-frac laboratory investigations and analyses, post-frac laboratory investigations and analyses, and then to integrate these results with field data to clarify and interpret the results of both the Phase I and II stimulation operations. Phase I and II stimulation operations. BACKGROUND CORE DATA
The objectives of the MWX core program are to provide a physical description of the reservoir and provide a physical description of the reservoir and to support well testing and-stimulation. Prior to MWX, there was very little specialized core data (e.g., restored state permeabilities as a function of water saturation, capillary pressures, etc.) available from tight sandstone formations such as the Mesaverde. In fact, many of the first such measurements were made on WWX core.