Abstract:

Drilling cost reduction is a major concern for geothermal operators and crucial to long-term geothermal development. Reducing drilling days and lowering per-foot costs are primary means to achieving this goal. Baker Hughes, Inc. (BHI) developed the Kymera hybrid bit with these goals in mind. The advantage of the Kymera bit technology versus standard tungsten carbide insert (TCI) bits is in the Kymera’s combination of roller-cone design and polycrystalline diamond compact (PDC) bit design. In trials at EnergySource’s Hudson Ranch II project, performance data from 12.25- in (311 mm) and 9.875- in (250.8 mm) Kymera bit runs are compared to TCI bit runs in offset wells. Performance is measured using a systematic approach comparing revolutions per minute (RPM) and rate of penetration (ROP) in both slide and rotating modes. Formation and depth are also factored into the performance data. The Kymera hybrid bit increased rates of penetration and total footage drilled per bit run, resulting in a significant reduction in cost per foot (CPF) versus standard roller-cone bits for a substantial cost savings.

Abstract:

In June 1991, a high-pressure/high-temperature well located in Hawaii kicked and unloaded at 3,476 ft (1,059 m). This well was estimated to have a possible bottomhole temperature of 650°F (343°C) and a reservoir pressure approaching 2,300 psi (15,858kPa). Immediate attempts to kill the well were unsuccessful, and the long process
of well control was started.

Besides the harsh geological and reservoir conditions encountered, the scarce availability of materials in a remote location and long distance transportation of necessary equipment figured heavily into the time delay of the final kill procedure of the well.

Regaining the control of the well in a remote, tropical environment was accomplished using state-of-the-art well control techniques. These techniques were applied through the operating company's careful coordination of service companies and personnel during a month period.

Abstract:

Scientific test well 58-32 was drilled to a depth of 7,536 ft (2,297 m) to obtain direct measurements on rock type, temperature, permeability and stress within the Utah FORGE reservoir. Well 58-32 encountered the top of the granite at about 3200 ft. (975 m) and the top of the FORGE reservoir at 6500 ft. (1980 m) based on expected formation temperature. Drilling tool selection kept the rate of penetration reasonable through the hard rock formation. The well was cased with 7-inch casing to a depth of 7,374 ft. (2,247 m). Core samples were collected near the top of the expected reservoir and at total depth, prior to running the casing in the hole. Injection testing and fall-off
pressures provide data on the permeability of the crystalline rock. Additional data were obtained, including a suite of geophysical logs, pressure and temperature surveys, and microresistivity image logs. An injection test was conducted in the open hole section of the well. The temperature surveys indicate over 350°F (175°C) at the reservoir depth, as required for the FORGE initiative enhanced geothermal systems laboratory. The drilling and testing of 58-32 well was completed deeper than planned, ahead of schedule and under budget.

Abstract:

We investigated the usefulness of the coal combustion by-products, Class C fly ash (C) and ClassF fly ash (F), in developing cost-effective acid resistant phosphate-based cements for geothermal wells. In the temperature range of 20-100 °C, sodium polyphosphate (NaP) as the acidic cement forming solution preferentially reacted with calcium sulfate and lime in the C as the base solid reactant through the exothermic acid-base reaction route, rather than with the tricalcium aluminate in C. This reaction led to the formation of hydroxyapatite (HOAp). In contrast, there was no acid-base reaction between the F as the acidic solid reactant and NaP. After autoclaving the cements at 250º, a well crystallized HOAp phase was formed in the NaP modified C Cement that was responsible for densifying the cement's structure, thereby conferring low water permeability and good compressive strength on the cement. However, the HOAp was susceptible to hot CO2 laden H2SO4 solution (pH 1.1), allowing some acid erosion of the cement. On the other hand, the mullite in F- hydrothermally reacted with the Na from NaP to form the anaclime phase. Although this phase played a pivotal role in abating acid erosion, its generation created an undesirable porous structure in the cement. We demonstrated that blending fly ash with a C/F ratio of 70/30 resulted in the most suitable properties for acid resistant phosphate-based cement systems.

Abstract:

Drilling cost and risk is the greatest impediment to global geothermal development. In the early 1990s, the use of lower cost slim holes was introduced for geothermal exploration. Although the industry was slow to adopt this method, slim holes are now commonly drilled and tested to evaluate geothermal resource potential. With the advancement of miniaturized instruments and other components, microbore exploration wells can reduce drilling cost and risk. Of critical importance in the use of a surrogate slim hole or microbore is the assumption that test results can be accurately scaled to larger, more expensive production bores to be completed after successful discovery of a resource. The accuracy of this scaling varies with bore diameter, resource conditions and the amount of scale up to larger sizes desired. Geothermal exploration wells are typically evaluated by discharging the well to surface equipment at atmospheric pressure to measure flow rate, enthalpy, and fluid composition. Reservoir characteristics are further evaluated by conducting injection tests, step-rate production tests, and pressure recovery measurements. However, low temperature resources or small diameter bores are often incapable of continuous, unassisted flow. In such cases, flow to the surface can sometimes be induced, or temporarily maintained, by air- or nitrogen-lift, or pumping, but these methods add significantly to the cost and complexity of the test operation. In addition, atmospheric flow tests require relatively large liquid storage facilities (sumps or tanks) or a nearby injection well, and may be limited due to steam and gas emission considerations, hazardous liquid composition, or water disposal restrictions. Using innovative test methods, microbore resource evaluation can be completed using injection, drill stem tests, and in situ chemical analysis. These methods require substantially less infrastructure and reduce the time required for resource evaluation.

Abstract:

Evaluation of cement bonding and zonal isolation is a challenge that the oil and gas industry continues to face as wells are drilled deeper within more hostile environments. The complexity of this task has increased as these wellbores have more challenging trajectories and being drilled in formations for which there is little and completions experience. In addition, cement slurries have become more complex with the addition of inert gases, microspheres, non-traditional liquids, and many other additives designed to improve the cement sheath quality. These slurries require non-traditional interpretation methods to effectively evaluate the cement sheath because older methods do not yield accurate results in these situations. This paper will present information concerning the existing cement evaluation logging tools, basic interpretation techniques, and an overview of the new, advanced methods for existing tools available from a variety of vendors in the industry.

Progress is continuously being made in the development of more effective cement evaluation tools and evaluation techniques. Standard cement evaluation logging tools consist of two major classes, sonic and ultrasonic. The standard cement bond log, segmented bond log, and radial bond log are all part of the sonic logging family. The ultrasonic family consists of tools with either a rotating transducer of a stationary array of transducers. This paper, however, will not focus on the hardware but will focus on the interpretation of available measurements and on facilitating optimized decisions using measurements from both families.

Advanced interpretation methods discussed in this paper broaden refine previously published methods in order to effectively evaluate wellbore conditions with the commonly available cement evaluation tools. The original processes developed in the early 1990s now incorporate a statistical variance mapping display for both the sonic and ultrasonic tools. The resulting variance image from the ultrasonic tools allows detection of minor changes in cement or fluid composition and aids in the interpretation of the pipe-to-cement bond. This technique provides a robust answer product helpful in diagnosing zonal isolation and highlighting channeling and quality of materials behind pipe for all cement compositions. It is also possible today to process and interpret non-standard sonic data, such as refracted monopole and flexural dipole from logging tools not specifically designed for cement evaluation.

Correct application of the newer interpretation techniques described in this paper can lead to fully evaluated cement sheath quality and distribution behind pipe. Several examples using the new advanced interpretation methods will be presented including comparisons (1) between a scanning ultrasonic tool and a radial bond tool (2) a sequence of evaluations using a cement bond log combined with an ultrasonic tool before and after several multiple remedial squeeze operations, and (3) also a comparison between two different scanning ultrasonic tools and a segmented bond tool. The final example shows a successful use of the new technique in a completion using titanium casing.