Some Economic Aspects of Water-Rock Interaction<xref rid="ch07fn1"><sup>1</sup></xref>

Water, through its unique and extreme properties, is the fundamental fluid genetically relating all mineral deposits in sedimentary rocks. Economically important mineral deposits in sedimentary rocks which are the result of natural water-rock interaction include petroleum and Mississippi-type lead-zinc deposits. Understanding of the origin of these deposits through water-rock interaction requires knowledge of the relations between hydrochemistry and hydrodynamics. The recovery of some of these mineral deposits involves man-imposed water-rock interactions, for example, during water flooding of petroleum reservoirs, in-situ steam injection into oil sand deposits and underground coal gasification. These man-imposed water-rock interactions may result in subsurface reactions w ich can reduce permeability, produce toxic or deleterious substances which require removal before reuse of the produced water, contaminate local potable groundwater, or cause problems in waste injection wells because of subsequent water-rock reactions. Although we understand some of the principles involved, it is clear that considerably more thought and additional research effort needs to be directed to these and other economic aspects of water-rock interaction.

Drilling waste sub-surface disposal, via injecting into a pre-defined suitable formation, known as Waste Injection (WI) technology; has been selected as a primary drilling waste management tool for the development of a major oil field in the Caspian Sea. The field development has been utilizing WI technology since the beginning of development drilling operations. The main drivers were proven track record of the technology, meeting "zero-discharge" environmental requirements and elimination of logistical complications associated with onshore transfer of drill cuttings. A dedicated WI well was drilled and the surface equipment package to process and inject drilling waste was installed on each drilling and production platform. WI failure was considered as one of the top ten risks to achieving full field production targets. Complexity of the injection horizon consisting mostly of massive mudstone, regional faulting and tectonic activity combined with a lack of local WI experience called for a detailed sub-surface engineering design and meticulous monitoring of the waste disposal domain. Continuous pressure monitoring was recognized as a critical approach in providing sub-surface assurance and timely identification of any early-warning signatures. Unique to these WI wells, the injection wells were equipped with down-hole pressure-temperature gauges for accurate pressure measurement and subsequent analysis. This paper discusses the execution, results and conclusions of monitoring this challenging project over a 5-year period, including subsurface complexities experienced during this period such as very high injection pressures, limited fracture-wellbore communication, enhanced solids settling, observation and explanation of long-term pressure trend behaviours. In addition solutions developed to prolong the life of the injection asset; including enhanced seawater "over-flush" injection strategy coupled with down-hole pressure measurement, careful monitoring and analysis.

Biologic activity can clog waste injection wells and produce gas in aquifers. Beneficial effects such as solubilization of particulate matter are also possible. Some organisms, particularly Protozoa, fungi, and bacteria of the biotic kingdom Protista, thrive under extreme conditions. Therefore, the potential for problems of biologic origin must be evaluated carefully in every situation. Exclusion of biota is to be expected only under the most hostile conditions. Versatility in adaptation to unusual environments and size limitations imposed by typical aquifer materials suggest that Protista will be the predominant biota in the waste injection regime. The composition, size, and activity of a protistan population depends upon many factors. These include temperature, pH, salt content, concentration and types of nutrients and micronutrients available, oxygen concentration, and aquifer lithology among other things. All chemical elements necessary for cell building such as carbon, nitrogen, phosphorous, sulfur, and numerous trace elements must be present. Past experience with artificial recharge wells suggests that public-health jeopardy by microorganisms introduced by injection of certain types of waste is not great. Bacterial travel in confined aquifers is negligible and survival time is short. Exceptions may exist in highly permeable strata. Microbial growth supported by nutrients in the injectant occurs near the well screen. Addition of disinfectants to control microbial growth may be useful but certain biocides may become nutrients under some circumstances. The biocide may be ineffective in the waste injection regime. Slime control measures must be carefully selected. End_of_Article - Last_Page 2083------------

Physical and numerical simulation of inter-fracture flooding in heterogeneous tight oil reservoirs

The unconventional use of coal to supplement Natural Gas (NG) in the power and chemical industry makes Underground Coal Gasification (UCG) an important technology in economically producing unconventional gas. Present day exploration and production technologies pave the way "from a potential to actual production". A 3D seismic survey has been applied in Southern Hungary for the site selection of UCG resource blocks, as well as in the design of the most optimal exploration drilling program. The latter exploration techniques directional drilled injection and production wells are planned in the coal seams to sustain the burning front. Wildhorse UCG Kft is a pioneer in the design and introduction of the environmentally friendly coal based syngas for electric power generation in East-Central European countries. Additionally the syngas can be utilised to supplement national gas supplies as alternative fuel gas. In this presentation the role of 3D seismic is discussed in defining the "Mecseknádasd UCG project". Wildhorse UCG Kft completed 3D seismic measurements in April and May of 2011 in the Mecseknádasd UCG project area. The goal of the seismic measurements is to image and clarify the structural conditions and to reveal faults and other discontinuities in the coal formation explored previously by historic deep drilling in the area. The processing and interpretation of the survey results have been performed together by GEOMEGA Kft and Wildhorse UCG Kft and the presentation covers the complete survey results of the Mecseknádasd UCG exploration project: –Newly processed results of historical boreholes–Results of the seismic survey–Geological and geophysical results of new boreholes–Possibilities for upgrading the geological model with integrated interpretation by using the available geophysical data (seismic, new boreholes, historic boreholes)–The possibilities for the application of 3D seismic survey in the design process of the UCG technology and to monitor the UCG engineering process

Electrical resistivity and passive acoustic techniques have been used to remotely monitor the location of the reaction zone in an in situ coal gasification experiment conducted near Hanna, Wyoming. Thermal measurements were made to provide diagnostic information, to aid in provide diagnostic information, to aid in understanding the process, and to evaluate the effectiveness of the remote techniques. From a comparison of field resistivity data and model calculations the path of the reaction zone was determined. Source locations in the overburden for acoustical events during gasified area. The feasibility of these remote techniques to monitor in situ coal gasification has been demonstrated. Introduction The Laramie Energy Research Center (LERC) conducted an underground coal gasification (UCG) experiment at a site near Hanna, Wyoming from March 1973 to March 1974. This experiment, Hanna I, demonstrated that western coals were suited to underground gasification and that it was possible with only air injection to produce 125 Btu/scf gas for an extended period of time. Sandia Laboratories participated during this experiment and demonstrated that resistivity, acoustic, and temperature measurements could detect effects associated with the process. Phase 1 of a Hanna II experiment was conducted Phase 1 of a Hanna II experiment was conducted May through August, 1975. This phase was designed to determine in situ permeability, to examine pneumatic and combustion linking of wells and to conduct a brief gasification test. Phases 2 and 3, the linkage and gasification Phases 2 and 3, the linkage and gasification between a pair of wells and evaluation of a linedrive concept for areal recovery were conducted from April through July, 1976. These experiments have been based on the linked vertical well concept. In this concept communication between the injection and production (recovery) wells through the coal seam production (recovery) wells through the coal seam is accomplished by thermal linking. In this method the coal is ignited at the production well, while air is supplied at the injection well. The combusion front follows natural fissures toward the source of oxygen and develops a localized permeable path of hot, carbonized coal. When the permeable path of hot, carbonized coal. When the combusion front reaches the injection well there is an abrupt drop in injection pressure to less 50 psi with a significant increase in the flow rate. psi with a significant increase in the flow rate. Once linkage is established combusion mode in which the combusion front moves in the direction of air flow along the permeable link. LERC is now conducting a second UCG experiment which has been designated as Hanna II. Sandia Laboratories' participation in the Hanna II series of experiments is to develop and evaluate instrumentation techniques that will provide subsurface diagnostic information as an aid toward understanding the retorting process and to study commercially practical techniques to observe the subsurface burn with remote monitoring.

The basal sandstone is a poorly sorted, well indurated sandstone, which lies below the Conasauga Group and above the Precambrian crystalline rocks. It is an unknown resource defined by limited data, with only 14 data points (wells) for the entire State of Tennessee. The basal sandstone is thought to occur throughout most of the State west of the Valley and Ridge province at depths of generally more than 5,500 feet below land surface. The basal sandstone probably does not receive significant vertical recharge because the sandstone is overlain by such a thick sequence of flat-lying, low-porosity lower Paleozoic carbonates and shales. Data from two sites indicate that the rocks of the basal sandstone have relatively low porosity and permeability. The concentrations of dissolved solids in water from the basal sandstone range from less than 40,000 milligrams per liter to more than 200,000 milligrams per liter. The basal sandstone is not being used as a source of drinking water because of its great depth, the presence of shallower sources of drinking water, and possible concentrations of more than 10,000 milligrams per liter dissolved solids throughout its area of occurrence.

Relevance. The need to predict deformations of the earth's surface near injection wells pumping liquid radioactive waste, as well as their stability when pressure changes in the well during injection. Aim. To perform a numerical analysis of the stress-strain state of the structural elements of the injection well and the surrounding rock mass using the finite element method, to determine the magnitude of the deformations of the earth's surface and define the stability of the well and the strength of the cement stone at the maximum injection pressure of liquid radioactive waste. Objects. Near-wellbore zone of one of the injection wells located at the liquid radioactive waste injection site. Methods. Numerical finite element method for calculating the stress-strain state of the near-wellbore zone, taking into account the distribution of the elastic properties of rocks along the simulated section and the main structural elements of the well. Results. The authors have developed the numerical finite element scheme of one of the injection wells, taking into account its main structural elements, as well as allowing you to set the pressure distribution within the exploited horizon during the injection of waste. In the model, the distribution of the elastic properties of rocks along the section was set, taking into account their lithological features. The authors carried out the analysis of the hydrodynamic parameters of the well operation and determined the maximum pressure at the wellhead during the injection of liquid radioactive waste, equal to 1.71 MPa. Numerical modeling of the stress-strain state of the near-wellbore zone was performed in two stages: for the conditions of an idle well and taking into account the pressure distribution at the maximum waste injection pressure. The distribution of vertical displacements at the level of the earth's surface, as well as at the top of the operational horizon at the maximum pressure of liquid radioactive waste injection, is obtained. It is shown that at such a pressure, the largest uplifts will be: on the earth's surface – 4.5 cm, on the top of the production horizon – 11 cm. An assessment of the stability of the well structural elements at a maximum waste injection depth of 280 m, corresponding to the maximum pressure, showed that the stresses in the well and cement stone are much lower than the values that can lead to their violation. For cement stone, the safety factor at a maximum pressure of 1.71 MPa was equal to 7.8.

Mineral and rock deposits are unevenly distributed throughout the world and throughout most countries where they occur. Thus trade or exchange began early in human society to overcome this uneven distribution. Those who exploited rock and mineral deposits in the ancient world were limited to extracting material from surface or very near-surface deposits. The nature and quality of an exploitable deposit reside ultimately in the kind, amount, and properties of the minerals and rocks that comprise the deposit as well as the ease with which the desired material can be extracted. There is only a limited primary literature on recovery/extraction of mineral raw materials in antiquity. Wilsdorf (1952) and Weisgerber (1976) have studied Corinthian clay tablets and vases for mining and metallurgical displays. Apparently, only clay mining and ore smelting are depicted.

This paper describes the development, validation, and application of a numerical model that simulates solids injection operations for soft rock reservoirs. There exist many classical fracturing models that are good tools for evaluating solids injection operations in formations with competent rocks. However, when evaluating solids injection operations in soft rock formations such as unconsolidated sand, these classical fracturing models generally fail to provide reliable evaluations. In this paper, a solids injection model was specifically formulated and developed for solids injection in soft rock formations. The development of the proposed model for soft rock formations is based on the Biot's self-consistent theory, plasticity theory, and fracturing and liquefaction criteria for rock. The predictive capability of the developed model was verified against field data from waste injection wells in soft rock formations. The validated model was used to evaluate solid waste disposal for cases of various injection operation conditions in a soft rock reservoir. Results show that the solids injection model provides an effective tool to assess the maximum injection volume allowed per well, the area of influence of injection, the bottom-hole injection pressure, and the vertical containment of the injected fluids and solids. Simulation results for representative cases are presented. The model can also be useful for quantitatively assist in identifying feasible solids injection mechanisms in soft rock reservoirs.

Deep well injection operations represent one of the available methods of waste disposal. Approximately 14 million tons of liquid wastes in United States were injected into the subsurface in 1988 [Hanson, 1989]; the majority of the hazardous waste injection wells were located along the Gulf Coast and near the Great Lakes. The location of these wells often coincided with areas where oil and gas related explorations had been performed; the exploration efforts provided abundant data on the subsurface formations which were found to be environmentally safe for injection of wastes. The largest user of hazardous waste injection wells in the United States was noted to be the chemical industry [U.S.EPA, 1985]. Deep well injection programs are used in several countries including Canada and Greece. The environmental regulations for hazardous wastes which are presently being drafted in Turkey envision the use of deep well injection programs.

This paper presents the results of an extensive study and field test carried out at the site of Prudhoe Bay's four oily waste injection wells. The field work was part of an overall environmental assessment intended to: (1) confirm earlier results indicating that no fluid communication was occurring with the permafrost; (2) determine optimum conditions for the disposal of waste in the presence of hydraulically-induced fractures; (3) substantiate that an increased injection pressure could be safely implemented. A three day injection test, including a step-rate stage, was carried out. Data collected included surface and downhole pressures, in-si tu stress measurements, and monitoring of ground surface deflections and wellbore hydraulic impedance. Radially symmetric surface tilt patterns showed that the test well connects to a horizontal fracture of 60-foot radius. Wellbore impedance indicated that a horizontal fracture with 9-18 foot radius communicated with the well. Integration of rock mechanics, historical information, and the collected data provided a clear picture of what was occurring underground. The different evaluation techniques showed consistent results as reflected in the estimated fracture size, placement and damage zone properties.

Outlook for the coal industry and new coal production technologies

Underground coal gasification (UCG) is a technique used for the exploration of energy from unmineable coal seams that are uneconomical to explore by conventional mining techniques. During UCG, associated rocks (mostly sandstone and shale) are being exposed to a temperature range of 900 °C-1000 °C. This thesis aims for a micro-nano-scale investigation of the structural change of sandstone with the support of micro-CT techniques along with some experimentation in confined and unconfined conditions. The in-depth investigation includes the observation of microstructural responses, pore-space evolution and pore-network models as a function of temperatures and provides valuable information about the pore-network attributes.

A simplified model is described which will indicate the economic value of the raw product gas from an experimental underground coal gasification test on a real-time basis in order to aid in the optimization of the process during the course of the test. The model relates the properties of the product gas and the injection gas to the cost of producing each of five potential commercial products. This model is being utilized to evaluate data during the Gulf-DOE underground coal gasification test at Rawlins, Wyoming in the fall of 1981.