Djebbar Tiab

Enhanced Reservoir Description: Using Core and Log Data to Identify Hydraulic (Flow) Units and Predict Permeability in Uncored Intervals/Wells

Understanding complex variations in pare geomet~ within different Iithofacies is the key to improved reservoir description and exploitation. Core data provide in~ornration on various depositional and diagenetic controls on pore geometry. Variations in pore geometrical attributes in rum, de

Point-Load Strength Test

ne the existenceof distinct zones(hydraulic units) with similar f?uid-jlow characteristics. Classic discrimination of mck types has been based on subjective geological observations and on empirical relationships between the log of permeability versus porosity. Howevec for any porosity within a given mck type,permeability can vary by several orders of nragnitnde, which indicates the existenceof severalflow units. In this papec a new, practical and theoretically correct methodology is proposedfor identi

Analysis of pressure and pressure derivative without type-curve matching: Vertically fractured wells in closed systems

cation and characterization of hydraulic units widtin mappable geological units (facies). The technique is based on a modified Kozeny-Carmen equation and the conceptof mean hydraulic raditis. The equation indicatesIhat for any hydraulic unit, a log-log p!ot of a “Reservoir Quality index,” (RQI), which is equal to 0.0314 ~. versus a “Normalized PorosityIndex” (+=) which is equal to WI-W should yield a straight line with a unit slope. 7he intercept of the unit slope line with +Z = 1, designated as the “FIow Zme Indicator” (M), is a unique parameter for each hydraulic unit. RQI, 4, and FZI are based on stressed potvsity and permeability data measuredon core samples.

Analysis of pressure and pressure derivative without type-curve matching — Skin and wellbore storage☆

This chapter discusses that the point-load test is designed to provide a rapid, portable index test for rock strength. A sample is compressed among solid steel cones that generate tensile stress normal to the axis of loading. The point-load strength index (Is) is defined as the load measured at the point of failure of the rock sample (Fa) divided by the distance among the conical platens at the moment of failure (Ls). Empirical testing has revealed a direct correlation of the Is index to the uniaxial compressive rock strength when the test is performed using a cylindrical core the length of the core is greater than 1.4 times the diameter of the core and the diametral test is performed.

Porosity and Permeability

Abstract This paper discusses a new technique for interpreting log-log plots of pressure and pressure derivatives of vertically fractured wells in closed systems without using type curve matching. For the uniform flux fracture case, pressure derivative plots for various x e x f ratios reveal three dominant flow regimes. During early times the flow of fluids is linear and can be identified by a straight line of slope 0.5. The linear flow line is used to calculate the half-fracture length. The infinite acting radial flow regime, which can be identified by a horizontal straight line, is dominant for x e x f > 8 . This flow regime is used to calculate permeability and skin. The third straight line, which corresponds to the pseudosteady flow regime, has a unit slope. This line is used to calculate the drainage area and shape factor. For the infinite conductivity fracture case, pressure derivative plots reveal a fourth dominant flow regime, called here the bi-radial flow . This flow regime, which can be identified by a straight line of slope 0.36, also can be used to calculate the half-fracture length and permeability.

Constant rate solutions for a fractured well with an asymmetric fracture

Abstract The current type-curve matching technique is essentially a trial-and-error procedure. A new technique for interpreting pressure tests using log-log plots of the pressure and pressure derivative versus time to calculate reservoir and well parameters without type-curve matching is presented. This paper concentrates on the interpretation of pressure tests in which wellbore storage and skin are present. Characteristic points are obtained of intersection of various straight line portions of the pressure and pressure derivative curve, slopes and starting times of these straight lines. These points, slopes and times are then used with appropriate equations to solve directly for permeability, wellbore storage and skin. A step-by-step procedure for calculating these parameters without type-curve matching for five different cases is included in the paper. The most important aspect of this new technique is its accuracy because it uses exact analytical solutions to calculate permeability, skin, and wellbore storage. The proposed technique is applicable to the interpretation of pressure buildup and drawdown tests and is illustrated by several numerical examples.

Analysis of Pressure Derivative Data of Finite-Conductivity Fractures by the "Direct Synthesis" Technique

The knowledge of the two properties—porosity and permeability—of rocks is essential to determine the types of fluids, amount of fluids, rates of fluid flow, and fluid recovery estimates in rocks. The porosity of a reservoir rock is defined as that fraction of the bulk volume of the reservoir that is not occupied by the solid framework of the reservoir. The porosities of petroleum reservoirs range from 5% to 30%, but most frequently are between 10% and 20%. Any porosity less than 5% is very seldom commercial, and any porosity more than 35% is extremely unusual. During sedimentation and lithification, some of the pore spaces initially developed became isolated from the other pore spaces by various diagenetic and catagenetic processes such as cementation and compaction. Thus, many of the pores will be interconnected, whereas others will be completely isolated. This leads to two distinct categories of porosity—total and effective—depending upon which pore spaces are measured in determining the volume of these pore spaces. In addition to being porous, a reservoir rock must have the ability to allow petroleum fluids to flow through its interconnected pores. The rocks ability to conduct fluids is termed as permeability. The permeability of a rock depends on its effective porosity and consequently, it is affected by the rock grain size, grain shape, grain size distribution (sorting), grain packing, and the degree of consolidation and cementation.

Pressure behaviours and flow regimes of a horizontal well with multiple inclined hydraulic fractures

This paper presents solutions for the pressure response on hydraulically fractured wells flowing at constant flow rate through an asymmetric vertical fracture. The pressure behavior of wells intercepting asymmetric fractures of both infinite and finite conductivity was investigated by solving numerically and analytically the mathematical model. The new solutions developed for the dimensionless wellbore pressure under production at constant flow rate are presented in terms of an asymmetry factor ξ. New curves for these systems were generated and the deviation from the classical solution was readily detected. Some qualitative criteria to interpret the intensity of this effect are provided. Results of our investigation indicated that at early times for fractures of moderate conductivity (CD<5) the characteristic slope of one fourth is present, except for cases of strong asymmetry (0.85<ξ≤1) where no evidence of straight line having one fourth slope was observed. However, it was also detected that at intermediate fracture conductivities (5<CD<50), even the case of complete asymmetry shows the characteristic slope of one fourth. It was also observed that as the asymmetry factor increases, the end of the bilinear flow occurs earlier. Our results are relevant in improving the fracture characterization of fractured wells.

Interpretation of stress damage on fracture conductivity

This paper extends Tiabs Direct Synthesis technique for interpreting the behavior of the pressure and pressure derivative data of a well intersected by a finite conductivity hydraulic fracture. In this technique log-log plots of pressure and pressure derivative data of a pressure drawdown or pressure buildup test are analyzed without using the type-curve matching or regression procedures. A log-log plot of pressure and pressure derivative versus test time for a fractured well in a closed system may reveal the presence of several straight lines corresponding to different flow regimes; bilinear flow, linear flow, infinite-acting radial flow, and pseudo-steady state flow. The slopes and points of intersection of these straight lines are unique and therefore can be used to calculate several well, reservoir and fracture parameters: permeability, skin factor, wellbore storage coefficient, fracture conductivity, half-fracture length, and drainage area. It is found that equations corresponding to the points of intersection are very useful in checking on the parameters obtained from the slopes, when the pressure derivative curve is not smooth. A new equation is derived for calculating (a) the half-fracture length in the absence of the linear flow regime straight line of slope 0.5 such as in the case of low conductivity fracture, (b) the fracture conductivity in the absence of the bi-linear flow line of slope 0.25, and (c) the skin factor in the absence of the infinite acting radial flow line such as in the case of a short test.

Effect of Pore Pressure on Conductivity and Permeability of Fractured Rocks

Hydraulic fractures have become a common stimulation application in the oil and gas industry, especially in tight formations. Published models assume the hydraulic fractures are vertical and symmetric. However, recent studies have shown that fractures are asymmetric and inclined with respect to the vertical direction and the axis of the wellbore. This paper introduces new analytical models that can be used to investigate the pressure behaviour and flow regimes of a horizontal well with multiple hydraulic fractures. The hydraulic fractures in this model could be longitudinal or transverse, vertical or inclined, symmetrical or asymmetrical. The fractures are propagated in isotropic or anisotropic formations and considered having different dimensions and different spacing. These models are solved using MATLAB codes to create several analytical solutions for the flow regimes that are expected to develop in the vicinity of the wellbore. These solutions can be used to calculate various reservoir parameters, including directional permeability, fracture length, skin factors, and angle of inclination.