Browsing by Subject "Drill core analysis -- Computer programs"
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Item Computer determination and comparison of volume of clay derived from petrophysical and laboratory analysis(Texas Tech University, 1991-05) Causey, Gary LeeVolume of clay (Vcl) is the most critical parameter to determine in shaly sand analysis. Yet in many areas it is very difficult to determine volume of clay accurately. An overestimation of clay volume can result in effective water saturations (Swe) that are too low, thus making the reservoir look productive. Under-estimation of clay volume can result in effective water saturations that are too high, which can result in the bypassing of a productive zone. The underestimation and overestimation of volume of clay will also effect the calculation of effective porosities used to determine both effective water saturations and net pay. The FORTRAN program SHALE is designed to calculate volume of clay from 14 different petrophysical methods using up to 7 petrophysical logs. In the program, corrections are made for lithology, pressure, mud weight, borehole variations, and bed thickness. In addition, corrections can be made to convert old gamma ray logs to the corresponding API scale. Laboratory determined volumes of clay can be input into the program and each petrophysical volume of clay raethod can be statistically compared so that the user can select the petrophysical volume of clay method that best estimates the true volume of clay. Using the most accurate petrophysical method chosen, SHALE will then derive a linear transform equation that can be used to correct petrophysically derived volumes of clay. Laboratory determined volume of clay data from 7 samples from a lower Miocene deltaic sand in the Malay basin indicates that the gamma ray unconsolidated method was the most accurate in determining volume of clay. Laboratory determined volume of clay from 30 samples from the Cretaceous Olmos shelf sands in Webb County, Texas indicates that the gamma ray unconsolidated method was the most accurate in calculating volume of clay. However, in both the lower Miocene and Olmos sands, all petrophysical methods overestimated the volume of clay and required transforms to better estimate the volume of clay. The overestimation of volume of clay in both the lower Miocene and Cretaceous Olmos sands indicates that it is critical to calibrate petrophysically derived clay volumes to laboratory analysis.Item Computer-aided statistical analysis of the correlation between shaly sand analysis using old logs (pre-porosity): and modern logs : Pennsylvanian Hoover sand, Harper and Beaver counties, Oklahoma(Texas Tech University, 1995-05) Grim, Gerald KennethInaccurate calculation of water saturation and porosity can mean the difference in billions of cubic feet in recoverable gas hydrocarbons. In a "clean" formation of sand matrix, water, and gas, an accurate water saturation and porosity can be calculated from the petrophysical log response using the Archie equation. When shale is present, the Archie equation must be modified to compensate for the effect of clay minerals on log response. The more log suites available for a particular well, the more accurate and more complex the correction for shale becomes. Many possibly productive shaly sand zones discovered prior to the 1960's were logged with only electrical logging tools. The lack of additional log devices severely limits the number of shaly sand corrections that can be made to the Archie equation. Therefore, several old electric log shaly sand methods are compared here to the dual water shaly sand model as determined from a nearby modern log (electrical, gamma-ray, density and neutron logs). Four old electric log analytical methods were attempted: uncorrected Archie equation (automatic compensation), Archie equation with a volume of clay correction, no-porosity log dual water model, and ratio water saturation. Each of these methods generated effective water saturations which were compared to the effective water saturation determined by the dual water model performed on a neighboring well with modern logs. The total porosity and effective porosity determined from old logs and modern logs, were also compared. The formation chosen for this study was the Pennsylvanian (Virgilian) Hoover Sand located in the panhandle of Oklahoma. The Hoover Sand was chosen based on its lithology, structure, well spacing, and the availabÃlity of a mixture of modern logs and old logs. The Hoover Sand in the Mocane-Laverne field was discovered in 1955 and has been a productive gas sand for almost 40 years. The formatÃon consÃsts of laminated silt and sand deposited via density flows or turbidity currents onto a shallow, broad shelf area that covers approximately 98,560 acres. Because the Hoover Sand is bounded above and below by shale and pinches out horÃzontally into a shale, the trapping mechanism is a structural/stratigraphic combination. Thirty-two pairs of wells were evaluated in 4 foot intervals. Based on the comparisons between old log shaly sand methods and a "comprehensive" modern log shaly sand model (dual water), the uncorrected Archie equation (automatic compensation method) resulted in water saturations that closely approximated the results obtained from modern logs. These values can then be used to determine a more accurate value for recoverable hydrocarbons. The use of the volume of clay corrected Archie method determined effective water saturations which were consistently more optlmistic than the other methods. The ratio water saturation returned the poorest correlation to the effective water saturation as determined from the dual water method used on a modern log.Item Petrophysical study of the Glorieta-Clearfork dolomite in the Monahans Field, Ward County, Texas(Texas Tech University, 1992-12) Saha, SouvickCalculation of a reservoir's water saturation using the Archie equation requires the values for cementation exponent (m) and saturation exponent (n). Determination of these two parameters, particularly in carbonate reservoirs, is often difficult. Recently a new method (CAPE) for estimating m and n has been proposed by Maute et al. (1992). In the CAPE (Core Archie Parameter Estimation) method, m and n are determined by minimizing the error between laboratory derived water saturation (Sw (core)) and water saturation calculated by the Archie equation (Sw(Archie)). Because core data are often unavailable, the author substituted dielectric water saturation (Sxo(dielectric)) for core-derived water saturation to determine m and n, and applied the technique to the Glorieta-Clearfork dolomites in the Monahans field, Ward County, Texas. The Permian (Leonardian) Glorieta-Clearfork dolomites in the Monahans field represents an upward-shoaling carbonate platform sequence. The predominant rock type is dolostone and the major mineral constituents are dolomite and anhydrite. Petrographic analysis reveals mainly intercrystalline/intergranular pore geometry with minor vuggy/moldic porosity. In this study the author applied three techniques: (1) non-linear, (2) linear, and (3) m-porosity transform to determine m and n values that minimize the error (errorfunction) between Sxo (dielectric) and Sxo (Archie). Two mporosity transforms were established, but the transform that represented the majority of the data (85%) was used to derive m values. Using data from the Glorieta-Clearfork dolomite in the Monahans field, the non-linear method resulted in the minimum error between Sxo (dielectric) and Sxo (Archie). The m and n values determined by the non-linear and linear methods probably do not represent physical rock characteristics but are only values that minimize the error functions. In contrast, m and n values determined by the mporosity transform method should represent physical attributes of reservoirs such as pore geometry or wettability. In order to further reduce the error between Sxo (dielectric) and Sxo (Archie), m and n were approximated by mathematical functions (polynomial and Fourier series) to model the vertical variation of m and n in the reservoir (variable m and n method). This variable m and n method based on a Fourier series resulted in the greatest errorreduction when compared to the non-linear method. After determining m and n values that result in the minimum error between Sxo (dielectric) and Sxo (Archie), these values can then be used to calculate the water saturation in the uninvaded zone (Sw).