Influence Of Overburden Pressure Research Articles

Influence of Overburden Pressure on Imbibition Behavior in Tight Sandstones Using Nuclear Magnetic Resonance Technique

Abstract Imbibition under overburden pressure can simulate the imbibition behavior in reservoir conditions during hydraulic fracturing, about which the mechanism is still unclear. This study investigated the imbibition with overburden pressure using a nuclear magnetic resonance (NMR) displacement design. The main contribution of this study is that the initial imbibition rate under confining pressure can reflect the pore connectivity of reservoirs under overburden pressure and a method for appraising the pore connectivity under confining pressure was established. The tight sandstone samples were collected from the Upper Paleozoic Taiyuan and Shihezi Formations in Ordos Basin. The Taiyuan Formation presents the apparent double-peak structure from NMR spectra, and liquid fills into small pore preferentially as a whole. When the imbibition time is on a square root scale, the cumulative imbibition height at the initial imbibition period is not stable, which deviates from the linear principle, and the initial imbibition rate ranges from 0.077 to 0.1145. The Shihezi Formation shows a dominant peak structure from NMR spectra, and the liquid has no obvious filling order as a whole. When the imbibition time is on a square root scale, the cumulative imbibition height at the initial imbibition period also deviates from the linear principle, and the initial imbibition rate ranges from 0.0641 to 0.1619.

ABOUT SKIN EFFECTS OF OBLITERATION AND COMPRESSION

There should be factual information on the skin-effect on each well that reduces oil and gas production for operational work, as well as for the calculation of oil and gas deposits or for the design of field development. Well development and operation experience shows that contamination of the bottomhole zone and deterioration of leakage properties may be associated not only with collution, but also with events such as obliteration and rock compaction. Thus, during the operation of wells, events such as shrinkage and retention of microcatches and capillaries in fractured-porous formations over time are inevitable. The impact of this factor must also be taken into account, as the method of impact on the bottomhole zone and the compression of formations under the influence of overburden pressure. Such events should be attributed to the factors that increase the resistance of the bottomhole zone and should be studied as an object of research. The discovery of deep hydrocarbon deposits and their development experience show that failure to take into account the deformation of the porous medium as a result of a drop in formation pressure during development reduces the reliability of development parameters. The commissioning of deeper oil, gas and gas condensate fields requires the interpretation of mining data and the development of methods for hydrodynamic research of wells, taking into account the density of layers. Compression of stratified formations with significant deformation during development and the impact of this effect on the development process significantly improves the theory of filtration, taking into account the compression of formations in gas-hydrodynamic modeling, interpretation of mining data and other issues. In order to study and assess the skin effect associated with obliteration and compression, it is necessary to analyze real well data during the operation of productive oil layers. Three of them used the concepts of "normal" production, productivity, as well as the lowest production and productivity of the oil well in operation, and analyzed the performance of the well for several years. This analysis is based on the fact that production changes from depression to exponential law. The article puts forward a version of the existence of obliteration and compression skin effects along with colmatation in a practical example, and a methodology has been developed and tested to determine them. It has been found that during real depressions for collectors, these skin effects (So-c) may be greater than the colmatation skin effect (Scol). However, in practice, the value of the Scol is not accurately determined by one measurement based on the pressure build-up curve. The skin effect of obliteration and compression (So-c) is simply often negligible. In order to assess the skin-effect of colmotion and obliteration-compression in the reduction of well production, calculations were made at the values of the contour radius Rk = 150 m and well radius rq = 0.1 m. As a result, it was determined that the production of wells can be reduced by 64% due to the obliteration-compression effect. This is about 10 times more than skin effect of colmotation. It was determined that the skin-effect of obliteration and compression had a greater effect on productivity should be taken into consideration when forecasting well production. Keywords: obliteration, skin effect, collector, productivity, production, pressure

Influence of overburden pressure and soil rigidity on uplift behavior of square plate anchor in uniform clay

A numerical study incorporating three-dimensional Eulerian large deformation finite element analyses is performed to investigate the pullout process of horizontal square plate anchors in both hypothetical weightless soil and soil with self-weight. The validity of the numerical model is established through verification against published experimental and numerical results. The failure mechanisms during the pullout process under different conditions are then investigated. Three types of failure mechanism are observed; of which only two have been reported in the literature. The third mechanism identified in this study, which is a partially localized flow mechanism, is operative when the soil overburden ratio is not high enough to mobilize the full flow mechanism. The influence of soil self-weight is directly investigated by incorporating the density of the soil in the finite element model and maintaining the gravitational acceleration field throughout the analysis. The critical overburden ratio corresponding to the full transition to a localized plastic flow mechanism is identified in this study. The effect of the soil rigidity index (E/su) on the anchor uplift capability has not been systematically investigated in earlier studies. Contrary to the general failure mechanism and the full flow mechanism described in the literature, the capacity factor corresponding to this new mechanism increases with increasing E/su. The capacity factors for square plate anchors corresponding to different anchor embedment ratios, overburden ratios and E/su are provided in the form of design charts.

Prediction of fracture gradient from formation pressures and depth using correlation and stepwise multiple regression techniques

The purpose of this study is to apply a correlation analysis in conjunction with stepwise multiple regression statistical techniques to fracture gradient, overburden pressure, pore pressure and true vertical depth of field determined data of oil/gas formation for investigation of correlation that exists between these properties. Offshore data from operators were obtained and a new fracture–gradient correlation procedure, based on a statistical analysis in normally pressured formations with depth, was applied. Based on the statistical analysis results, multiple prediction equations were developed for estimating fracture gradient from formation overburden pressure and pore pressure. The technique is ideally suited for implementation on a personal computer spreadsheet package. The tested data reveals that the influence of overburden pressure appears more noticeable than pore pressure, while true vertical depth presents no significant correlation. The result of this study was presented in the form of correlation coefficient models and predictive models.

Influence of Overburden Pressure on Soil–Nail Pullout Resistance in a Compacted Fill

Soil nailing has been widely used in many places in the world in the last two decades because of its technical and economical advantages. The nail–soil interface shear strength is an important parameter in soil nail design. This parameter is governed by a number of factors, among which the influence of the overburden pressure (or soil depth) is the most controversial. There are differing views concerning the effect of overburden on the nail–soil interface shear strength. In order to examine the influence of the overburden pressure, a series of laboratory pullout tests on soil nails installed in compacted completely decomposed granite fill have been conducted using two pullout boxes. Numerical simulations have also been carried out and the results are compared with the pullout test data. The procedures of the pullout tests and new features of the pullout boxes used are briefly described. Changes of the vertical stress in soil close to the nail throughout the course of soil nail installation and pullout are...

Frost susceptibility related to soil index properties

The analysis of frost-heave data on several soils confirmed that segregation potential, hence frost susceptibility, of saturated soils was best related to the average size of the fines fraction, the specific surface area of the fines fraction, and the ratio of the material's water content to its liquid limit. The influence of overburden pressure can also be accounted for by an empirical relationship between the segregation potential, the average size of the fines fraction, and the compressibility index of the soil. The segregation potential was also proportional to the relative fines content in soils where the fines do not completely fill the voids of the coarser fraction. This study led to the development of a new frost-susceptibility assessment methodology based on simple geotechnical routine soil index testing that was validated on a highway site on frost-susceptible subgrade till.Key words: frost heave, index properties, criteria, soil, segregation potential.

The use of stress-strength relationships in the assessment of tunnel stability

A method of predicting probable ground behaviour considering stress-strength relationships is presented. The influence of overburden pressure and rock mass strength which are implicitly used in calculating the Q value (Q-system of Barton et al. 1974) are explicitly considered in forecasting stability problems in tunnels. In particular, squeezing, spalling and loosening of the rock mass in tunnels constructed through the complex geological set-up of the Himalayas have been analysed. A new Q-system correlation which takes into account the dimension of an opening is suggested for evaluating support pressures in squeezing ground.

Effects of temperature and pressure on frost heaving

Concerning the influence of overburden pressure on the relation between rate of heave and cold-side temperature, studies show that the heave rate for various overburden pressures tends to converge as the cold-side temperature is lowered, and the maximum rates of ice accumulation at low pressures occur at temperatures closer to 0°C than do those at high pressures. Evidence is also presented that a heaving zone is involved in frost action rather than a freezing plane. This zone extends over an increasingly greater temperature range and distance as the overburden pressure is increased. Using the concept of a heaving zone leads to a method for calculating the heave-decrease curve. The period of heave decrease is marked by a rapidly increasing ice segregation ratio and a reduced frostpenetration rate.

Compressional velocities from multichannel refraction arrivals on Georges Bank—northwest Atlantic Ocean

Velocities were obtained from unreversed, refracted arrivals on analog records from a 48‐channel, 3.6-km hydrophone cable (3.89 km from the airgun array to the last hydrophone array). Approximately 200 records were analyzed along 1500 km of ship track on Georges Bank, northwest Atlantic Ocean, to obtain regional sediment velocity distribution to a depth of 1.4 km below sea level. This technique provides nearly continuous coverage of refraction velocities and vertical velocity gradients. Because of the length of the hydrophone cable and the vertical velocity gradients, the technique is applicable only to the Continental Shelf and the shallower parts of the Continental Slope in water depths less than 300 m. Sediment diagenesis, the influence of overburden pressure on compaction, lithology, density, and porosity are inferred from these data. Velocities of the sediment near the water‐sediment interface range from less than 1500 m/sec on the north edge of Georges Bank to 1830 m/sec for glacial deposits in the northcentral part of the bank. Velocity gradients in the upper 400 m range from [Formula: see text] on the south edge of the bank to [Formula: see text] on the north. Minimum gradients of [Formula: see text] were observed south of Nantucket Island. Velocities and velocity gradients are explained in relation to physical properties of the Cretaceous, Tertiary, and Pleistocene sediments. Isovelocity contours at 100-m/sec intervals are nearly horizontal in the upper 400 m. Isovelocity contours at greater depths show a greater difference from a mean depth because of the greater structural and lithological variation. Bottom densities inferred from the velocities range from 1.7 to [Formula: see text] and porosities range from 48 to 62 percent. The most significant factor controlling velocity distribution on Georges Bank is overburden pressure and resulting compaction. From the velocity data we conclude that Georges Bank has been partially overridden by a continental ice sheet.

Fissility in Argillaceous Rocks: ABSTRACT

A study was undertaken to determine the cause of fissility and non-fissility in argillaceous rocks such as shale and underclay. The orientation of clay flakes in samples of Pennsylvanian shales and underclays was studied by using an X-ray diffractometer. It was found that clay mineral orientation is a factor in determining the degree of fissility of an argillaceous rock. Sedimentation experiments were run in the laboratory to study the influence of various factors upon the development of fissility. Clay materials of various concentration were sedimented in different cation concentrations. Regardless of clay or cation concentration, the clay material flocculated. Various amounts of silt were sedimented on top of the flocculated clay masses. Preferred clay mineral orientation was produced in clay material on which silt was rapidly sedimented. Random orientation was preserved in clay material which had dewatered before a small amount of silt was applied. Samples of clay were also compressed to study the influence of overburden pressure and water content upon the development of fissility. It was found that clay with a lower water content showed less reorientation due to compression than clay with a higher content. This fact suggested that the amount of liquid water relative to rigid water in the clay-water system at the time of application of overburden pressure may be an important factor in facilitating the orientation of clay flakes. It is concluded that preferred orientation of clay flakes commonly found in fissile shales may have resulted from compression of flocculated clay mud under conditions of rapid sedimentation. The random clay mineral orientation present in underclay may have resulted as a flocculated clay accumulated under overburden pressure. Thus the presence of absence of fissility in argillaceous rocks may depend upon the amount of overburden pressure applied to the newly deposited clay material and the amount of liquid and rigid water in the clay at the time of application of pressure. End_of_Article - Last_Page 540------------