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.

Abstract:

This paper presents a simple method of designing a suspended fracture of a desired conductivity. The design balances volume, pump rate, and fluid loss to get the desired length. A sand schedule then is calculated to give the optimum flow capacity. The fluid and sand used to build the excess flow capacity in an equilibrium pack design are used to obtain better vertical coverage and deeper penetration.

The method is most applicable to low permeability reservoirs where the conductivity of an equilibrium pack is not needed. In this type of reservoir, the surface area of a fracture face will give up only a finite amount of fluid. It is not necessary for the fracture to have a greater flow capacity to the well­ bore than the formation has into the fracture. The design also can be applicable in some massive zones where the permeability is higher, but the cost to build an equilibrium pack is prohibitive. With a suspended pack design using the same amount of fluid, a deeper fracture with 100% vertical coverage can be obtained. In many cases, this produces a higher productivity increase with a lower cost.

The design centers on the sand concentration of the sand slurry in a finite number of segments along the fracture length. Using a compound-interest formula, the approximate amount of fluid that has leaked off in each segment is calculated. Then, with the final desired slurry concentration known, the sand concentration needed in each segment when pumped can be calculated.

The design is done in 10 easy steps with a program written for a TI-59 that will do almost all of the calculations included. The design is not applicable to every well, but where it does apply it can (1) give better vertical coverage, (2) get deeper penetration, (3) reduce costs for the same productivity increase, and (4) reduce the amount of load to recover.

Abstract:

One of the key findings of the 2014 Petroleum Practices Technology Transfer Committee report led by National Renewable Energy Laboratory (NREL) was that the geothermal drilling process lacked proper data collection while drilling, data integration, and analysis of such data. The lack of this essential engineering, well planning, and construction tool seemingly adds a significant amount of time to the 12-day additional non-productive time (NPT) at average per well while drilling geothermal wells versus oil and gas wells out of the 42 wells studied of the comparable order of magnitude of construction complexity. Geothermal Resource Group (GRG) has recently been involved in the construction of a well for a unique domestic enhanced geothermal system (EGS) observation drilling project (FORGE Utah) and was able to implement a hydraulic surface torque data collection system on a mechanical rig to analyze the MSE (mechanical specific energy). This paper presents the collection, evaluation, and the post-mortem comparison of the MSE to the drilling history, parameters, and changes in general lithological structures of this well.

Abstract:

Loss of circulation while drilling a Geothermal well is very common, because typically geothermal formations are highly fractured or poorly consolidated; these conditions inherently lead to lost circulation. These potential ‘thief’ zones may allow drilling fluid (and later, casing cement slurry) to enter the formation instead of circulating back to surface

When losses occur, lost circulation material is typically applied first, followed by cement plugs. Lost circulation cement plugs are used to isolate the well bore from highly porous or fractured zones that are under-pressured.

Cement plugs may be used to seal lost circulation zones, allowing drilling to continue through and below the loss zone with good returns to surface. Innovative drift plug technologies may be used in place of conventional balanced plug methods, achieving better results in severe loss zones.