Evaluation Of Bottomhole Treatment Pressure for Geothermal Well Hydraulic Fracture Stimulation
Authors: Morris Charles W. , Sinclair A. Richard 
Published: December 2007

Summary
The geothermal reservoir well stimulation program (GRWSP) group performed a large hydraulic fracture treatment in the well Baca 20, located in the Union Geothermal Co. of New Mexico Redondo Creek project area in north-central New Mexico. This is the highest temperature well in the world to be successfully fractured with proppant to date. The treatment involved use of a large cooling water prepad followed by a high viscosity fracturing fluid carrying sintered bauxite as the proppant. A nonproductive 240-ft [73-m] openhole interval, with a formation temperature of 520F [271C], was selected and isolated for the job. Poststimulation tests and analyses indicate a fracture was successfully created with a large vertical height near the wellbore. A number of fracture evaluation techniques were used to analyze the job, including some that are unique. Among these techniques were fracture mapping by monitoring the microseismic events caused by the fracture job, downhole treating pressure data analysis, temperature profiles, electric log surveys, production tests, and chemical tracer monitoring. The poststimulation fluid productivity of the treated interval was subcommercial, probably because of low formation permeability surrounding the artificially created fracture.

Introduction
The GRWSP, sponsored by the U.S. DOE, was initiated in Feb. 1979 to pursue industry interest in geothermal well stimulation work and to develop technical expertise in areas related to geothermal well stimulation activities. Republic Geothermal Inc. and its principal subcontractors (Santrol Products Inc., Maurer Engineering Inc., Ten-a Tek Inc., and Vetter Research) have completed seven full-scale field experiments to date. One experiment, performed on Oct. 5, 1981, is the Baca 20 well in New Mexico. The Baca reservoir lies within the Jemez Crater, Valles Caldera, and is defined by more than 20 wells completed to date in the Redondo Creek area by Union. The main reservoir, 4,000 to 6,000 ft [1220 to 1830 m] in thickness, is composed of volcanic tuffs with low permeability and a primary flow system of open fracture channels. In the Redondo Creek area, wells have encountered a high-temperature &gt;500F [260C]) liquid-dominated geothermal reservoir, but several wells have not been of commercial capacity, primarily because they lacked productive natural fractures at the wellbore. Baca 20 is near several of the better production wells in the Redondo Creek area. The original Baca 20 hole drilled toward the southeast was nonproductive. The well was redrilled to the northeast and marginally non-commercial production was obtained. The production interval of 3,322 ft [1013 in] extended from 2,505 to 5,827 ft [764 to 1777 m] in depth. A long-term production test performed in 1980 resulted in a stabilized flow rate of about 56,000 lbm/hr [25 400 kg/h] with a 56% steam fraction and a wellhead pressure of 116 psig [800 kPa]. The calculated permeability-thickness product was approximately 3,000 md-ft [915 md-m]. The primary in-flow zone was located at a depth of about 4,000 ft [1220 m] with deeper zones probably supplying some fluids. Unique pressure data were obtained during the treatment. These bottomhole pressure measurements, along with other techniques, were used to evaluate the fracture treatment and the ability of fracture design models to predict results in this type reservoir. The large hydraulic fracture job, described in the next section, was intended to continue development of the high-temperature stimulation technology needed for geothermal applications and to create the high-conductivity flow channel needed to make Baca 20 a commercial well.

Description of Stimulation Treatment
Baca 20 was originally completed as shown in Fig. 1A with a 9 5/8-in. [24.5-cm] liner cemented at 2,505 ft [764 m] and a 7-in. [17.8-cm] slotted liner hung at 2,390 ft [729 m] with the shoe at 5,812 ft [1772 m]. The 7-in. [ 17.8-cm] slotted liner was pulled, lost circulation zones cured using cement plugs, and then a 7-in. [17.8-cm] blank liner was cemented in place at 4,880 ft [1488 m] to isolate the desired treatment interval. Since the fracture interval was to be from 4,880 to 5,120 ft [1488 to 1561 m], a sand plug was placed from 5,827 ft total depth to 5,400 ft [1777 to 1646 m] and then capped with cement to 5,120 ft [1561 m]. The recompletion is shown in Fig. 1B. This particular 240-ft [73-m] interval was chosen primarily because the best production in the area has been found in comparable intervals near the bottom of the Bandelier tuff and because of its high reservoir temperature (520F [271C]).
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