Low-permeability Strata Research Articles

Hydromechanical modeling of tectonically driven groundwater flow with application to the Arkoma Foreland Basin

Deep groundwater flow can be driven by several mechanisms in sedimentary basins. In the case of evolving foreland basins, large‐scale compression and thrusting could develop abnormally high pressures in the foreland sag that would initiate transient fluid flow. The so‐called tectonic “squeegee” effect is thought to have caused basin‐wide migration of ore‐forming brines and hydrocarbons (Oliver, 1986). Two‐dimensional numerical models are developed here to quantify the role of compressional tectonics in driving regional fluid flow in the later stages of thrusting in a foreland basin. Poroelasticity theory coupled with regional groundwater flow form the basic elements of the mathematical model. We use the mathematical model to predict deformation and pressure dissipation in the unfaulted and nonfolded part of a foreland basin in front of a thrust belt as it is subjected to an instantaneous loading event. Sets of numerical experiments show that overpressure zones develop along the leading edge of the thrust belt near the loading front. Stress‐induced flow rates of the order of centimeters to meters per year are possible soon after compression of the foreland, and transient flow fields dissipate in about 103 and 104 years. Longer transients can exist in very low permeability strata. Large overpressures may be unable to buildup under conditions of gradual thrusting, as fluid pressures may dissipate too quickly. The general features of tectonically driven flow are also explored through a sensitivity study to consider effects of permeability, fault and stratigraphic heterogeneity, loading magnitude, and variations in rock compressibility. The sensitivity study is based mostly on numerical experiments. As these solutions suffered from stability problems in cases where bulk rock compressibility exceeded 10−9 Pa−1, some simple scaling arguments are used to extend the numerical results for squeezing of soft shale. One basin‐specific application to the Ouachita orogen suggests that tectonic squeezing could have caused transient flow systems with relatively large flow velocities in basal Cambro‐Ordovician aquifers. The volume of fluid expelled, however, is probably only a small fraction of the total brine volume needed to have formed the huge Mississippi Valley‐type lead‐zinc ore deposits fringing the northern margin of the Arkoma Basin on the Ozark Uplift.

Groundwater in the Quaternary deposits of Scotland: a valuable resource and a potential engineering hazard

Abstract The Scottish Quaternary strata contain a wide range of lithologies with an equally wide spectrum of hydrogeological properties. The more permeable material contains valuable groundwater resources which increasingly are being exploited for rural and island communities. However, the common juxtaposition of high and low permeability strata, the occurrence of confined groundwater in clay-capped gravels, and the risk of ground subsidence due to dewatering, together provide a range of potential Engineering problems and hazards.

Reservoir Description for Exploration and Development: What is Needed and When?: ABSTRACT

The biggest challenge for geologists, geophysicists, and petroleum engineers now and in the decades ahead is to significantly improve hydrocarbon recovery from all new and previously discovered reservoirs. Keystone of the methodology required to improve oil and gas production, as well as to evaluate and delineate new reserves, is a detailed reservoir description. This is a characterization of the reservoir and nonreservoir rock-fluid system that is appropriate in content and End_Page 1522------------------------------ detail for the particular stage of exploration and production. The type and amount of data required for a proper reservoir description are diverse, from several disciplines, and depend upon the stage of the reservoir's exploration and production cycle. The cycle is viewed as a continuous series of overlapping stages from discovery, through appraisal, planning, development, and reservoir management. The concepts and data needed to define and exploit reservoirs become more complex and quantitative as the production becomes more mature. Concepts, data, and models developed during the production phases, when reapplied to exploration, provide important guides to the explorationists for evaluating trapping elements, seals, reservoir quality, and risks in basin and wildcat evaluation. one looks at the question When is a reservoir description needed? the answer is simple. The need starts once a discovery is made and the discovery is being appraised as to the best estimates of hydrocarbon in place, recoverable reserves, and production rates. As a field or reservoir goes through its typical cycle of discovery, appraisal, planning, development, and reservoir management, a more complete description is both necessary and possible. A critical first step in the reservoir description process is recognizing any correlative reservoir subzones or layers and any intervening dense, impermeable, or low-permeability strata. Knowledge of the depositional/diagenetic processes controlling reservoir and nonreservoir rock is essential to determine one's ability and degree of confidence in correlating these units. Seismic sequence, lithologic, and fluid analyses, and well-documented outcrop studies can add significantly in establishing interwell correlations. Recognizing and mapping all vertical or horizontal fluid-flow barriers, as well as theif zones or zones of unusual permeability contrast and faults, are critically important to all recovery processes. Flow-test data dovetailed with knowledge of the reservoir and nonreserv ir framework based on geology/geophysics provide the best reservoir description of continuity/discontinuity. Structural and stratigraphic maps, cross sections, and fence-and-block diagrams convey the three-dimensional geometry, distribution, and continuity of the reservoir, nonreservoir, and aquifer. A variety of computer programs aid in preparing these illustrations. Isopach maps without the accompanying detail correlation sections have been the pitfall of many projects. Net-pay isopach maps drawn to provide the basis for determining hydrocarbons in place have tricked many petroleum engineers into believing a reservoir is more continuous, more homogeneous, and less stratified than it actually is. The importance of discontinuous shale barriers of limited areal extent on coning and the drainage of oil from a gas-invaded area illustrate the need to include shale dimensions in many types of recovery calculations and predictions. The recognition, selection, and description of reservoir units or layers, and the communication of this picture to the petroleum engineers are fundamental contributions and responsibilities of the geologists/geophysicists. A coordinated data-acquisition program can greatly improve the probabilities of correct assessments in discovery, appraisal, planning, development, and reservoir management. A good reservoir description designed to answer key reservoir performance questions is a fundamental tool. The incremental well costs to obtain adequate data for a reservoir description are very small compared with its value in improved recovery. The time to complete a reservoir description is before significant expenditures are planned and spent. Mathematical models and simulation of reservoir performance that do not have a realistic reservoir rock-fluid description are interesting, but are expensive exercises that potentially lead to inappropriate or incorrect management decisions. In exploration ventures, detailed reservoir-description studies made during the production stages provide the critical data needed by the explorationist to estimate reservoir and seal quality from seismic, well logs, and samples. End_of_Article - Last_Page 1523------------

Unsaturated Flow Conditions Beneath Lined Tailings Disposal Ponds: ABSTRACT

The uranium milling industry may soon dispose of substantial volumes of liquid waste in lined evaporation ponds. The potential for seepage through the low-permeable liner to contaminate ground water can only be assessed by analytic and numeric models. These models predict that, prior to reaching the water table, the liquid phase of the flow region will be unsaturated, except where perching may occur just above very low-permeable strata. None of the models actually simulate the effects of fractures in consolidated sediments. Because seepage beneath the pond is predicted to occur under partly saturated conditions, piezometers usually employed in ground-water studies generally will not be effective monitoring tools. However, neutron moisture probes, suction lysimeters, and tensiometers well known to soil physicists could be used to monitor unsaturated flow. Unfortunately, none of these instruments is likely to be highly reliable in fractured-rock environments or in heterogeneous sound rock with a limited number of monitoring points. In fractured and nonfractured rock the primary means of detecting seepage losses exceeding model predictions should be from a mass balance of inflow rate minus evaporation. Owing to unsaturated conditions beneath the pond, a well completed within the seepage zone could not produce water. This satisfies the NMEID mandate to protect future groundwater users after operations cease unless the seepage fluids contaminate the regional aquifer. Two- and three-dimensional numeric models show that seepage spreads beyond the perimeter of the pond; this may not be compatible with USEPA solid-waste-management regulations. End_of_Article - Last_Page 570------------

Geologic and Geophysical Study of Cerro Prieto Geothermal Field, Mexico: ABSTRACT

The Cerro Prieto geothermal field is near the southwestern margin of the Colorado River delta, Baja California. The subsurface stratigraphy at Cerro Prieto is characterized by complex vertical and lateral variations in lithofacies, which is typical of deltaic deposits. The geothermal production zone is not a uniform reservoir layer overlain by a laterally continuous top seal of low-permeability strata. The top of the geothermal-related hydrothermal alteration zone has a dome-like configuration which cuts across the sedimentary strata. Shales in the altered zone exhibit high densities and high resistivities on the well logs relative to those outside the zone. The geothermal producing intervals generally straddle or underlie the top of the altered shale zone. Sandstones in the hydrothermal alteration zone commonly have fair to good porosities (15 to 35% or higher), which have resulted from the removal of unstable grains and carbonate cement by solution. Open fractures are unusual in the altered zone, based on core description. While fractures may be an important contributor to local reservoir permeability, secondary matrix porosity and permeability are considered to be more important volumetrically in the Cerro Prieto reservoirs. Detection of geothermal anomalies in the Cerro Prieto region may be difficult from resistivity, magnetic, or gravity data. However, the occurrence of a reflection-poor zone coincident with the hydrothermal alteration zone suggests that the seismic reflection method may be a good approach to detecting these anomalies. Other types of geophysical data are necessary to eliminate alternate causes of reflection-poor zones on seismic profiles. End_of_Article - Last_Page 951------------

The influence of delayed drainage on data from pumping tests in unconfined aquifers

Equations are derived for the flow to a pumped well in an aquifer having uniform anisotropy and overlain by a low-permeability aquitard. The water-table is assumed to be located in the aquitard. Drainage from the capillary zone above the water-table is taken into account. The differential equation for the flow in the aquifer is identical with that derived in a previous paper. The formation constants may therefore be evaluated by using type curves as described in that paper. A well-known pumping test is reanalysed, using the equations in the present paper. It is shown that the time-drawdown curves can be explained only by the existence of a low-permeability stratum in the vicinity of the water-table. In this example the slow draining of the unsaturated zone above the water-table seems to be a significant factor in determining the shape of the time-drawdown curves.

Interaction of aquifers separated by low-permeability strata

Interaction of aquifers separated by low-permeability strata

Idealized Behavior of Solvent Banks in Stratified Reservoirs

Abstract One of the more important problems to be solved in designing a miscible flood is related to the size of the solvent bank used. Size of the bank may be critical to economic success. Too large a bank loses money; too small a bank may deteriorate and fail to maintain the miscibility needed for high recovery. An important factor is deterioration of a small bank is permeability channeling. In a highly stratified reservoir, solvent speeds ahead in the more permeable zones and mixes laterally with fluids bypassed in adjacent, low-permeability strata. Numerical solutions have been obtained for the differential equations that describe the movement of a slug through a two-layer system in which mixing occurs both in the direction of flow and transversely. The solvent slug is assumed to have the same density and viscosity as the resident fluid and the pushing fluid. These solutions have been verified by comparing them with similar concentration profiles obtained in the laboratory in a 36-ft stratified model packed with glass beads. The theoretical study revealed that when the dominant mechanism causing a bank to fail is lateral mixing the bank size needed for a given recovery may increase with length rather than decreasing as the square root of reservoir length, as suggested by one-dimensional mixing theory. From a comprehensive examination of the variables, a generalized correlation is developed that relates strata thicknesses, bank size, fluid velocity, mixing coefficients, system length and simple solvent-resident fluid phase behavior to the area miscibly swept. Introduction Miscible displacement, or solvent flooding, continues to receive widespread attention as a method for increasing oil recovery over that possible in conventional gas-drive or water-drive projects. A basic economic requirement in the application of such processes is the use of as little solvent as possible. A basic physical requirement is that enough solvent be used to maintain miscibility. Economics places an upper limit on the size of a solvent slug, and physical considerations establish a lower limit. Consequently, the practicality of any given miscible process requires that the economic limit be greater than the lower limit imposed by physical requirements. Procedures exist for determining the economic limit; however, procedures for determining realistic minimum bank sizes exist for only special reservoir situations. In the past, bank size has usually been selected on the assumption of a piston-like displacement for which only longitudinal mixing is important. This assumption leads to the favorable conclusion that bank size, expressed as per cent of pore volume, is inversely proportional to the square root of length. Collins considered the problem of transverse mixing of solvent with fluids in bypassed zones with the assumptions that no favored mixing occurs and that the concentration is uniform in the permeable stratum of interest.1 Lauwerier considered a mathematically similar problem in thermal recovery operations.2 Their work suggests the much less favorable conclusion that bank size could be directly proportional to the length or even higher powers of the length.