Electrical Properties of Reservoir Rocks

DETERMINING FLUID SATURATIONS Methods of Determining Saturations Conventional core analysis Capillary pressure measurements Well log analysis The remainder of the section will focus on conventional core analysis. Determination of fluid saturations from capillary measurements and log analysis will be discussed in later sections

OPENHOLE LOG EVALUATION Most abundant data for formation evaluation and determination of fluid saturations Well Log SP Resistivity This figure depicts the basic setup of the logging process. A wireline truck with a spool of logging cable is setup so that the sonde (measuring equipment) can be lowered into the wellbore. The logging tools measure different properties, such as spontaneous potential and formation resistivity, as the sonde is brought to the surface. The information is processed by a computer in the logging vehicle, and is interpreted by an engineer or geologist. From NExT, 1999

RHOMBIC PACKING OF SPHERES Electrical properties of reservoirs vary strongly with porosity and characteristics of the fluids in the pore space; usually, basic properties are determined assuming: “clean” reservoir rock (non-shaly) Sw = 1.00 (water saturated rock)

RESISTIVITY Resistivity Resistivity is an intensive rock/fluid property, and is a measure of (the inverse of) the electrical flow capacity of the rock Resistance is extensive and for linear, 1-D electrical flow, r=(R ·L)/A r electrical resistance,  R electrical resistivity, ·m L length of electrical flow path, m A cross-sectional area perpendicular to electrical flow path, m2

RESISTIVITY – DEFINITION OF THE OHM-METER From Halliburton (EL 1007)

RESISTIVITY Resistivity Resistivity is determined by measuring the voltage required to pass a measured amount of electrical current through the rock. For a rock cube with sides of 1 m, if electrical potential of 1 V (Volt) is required for current flow of 1 A (Ampere), then resistivity is 1 ·m (Ohm ·meter)

RESISTIVITY Resistivity Electrical resistance () is inverse of Electrical conductance (S, Siemens) 2  = (1/2) S Resistivity is the inverse of conductivity 2 ·m = (1/2) S/m

RESISTIVITY OF EARTH MATERIALS (1) Rock (2) Gas (3) Oil (4) Fresh Water (5) Salt Water Resistivity Conductivity Common Notations Ro = Resistivity of non-shaly rock saturated (Sw=1) with formation water (ohm-m) Rt = True formation resistivity (ohm-m) Rw = Formation water resistivity (ohm-m) From J. Jensen, PETE 321 Lecture Notes

ELECTRICITY AND EARTH MATERIALS Primarily, conduction is by ions in water Na+ and Cl- are very common Other monovalent ions K+ and OH- Bivalent ions, too: Ca++, Mg++ But, clays can conduct electricity also Modified from J. Jensen, PETE 321 Lecture Notes

Electricity and Earth Materials Water resistivity controlled by Ion concentrations Type of ions Temperature Chart GEN-4 to convert to NaCl equiv Chart GEN-5 for temp/resist for NaCl From J. Jensen, PETE 321 Lecture Notes

Factors Affecting Electrical Resistivity Resistivity of water Porosity of the formation, Pore geometry - tortuosity Lithology of the formation Degree of cementation, and Type and amount of clay in the rock From J. Jensen, PETE 321 Lecture Notes

Formation Factor Equation Saturation Equation Rock containing pores saturated with water and hydrocarbons Formation Factor Equation Non-shaly rock, 100% saturated with water having resistivity, Rw Rt Cube of water having resistivity, Rw = 20% Sw = 20% Ro = 20% Sw = 100% Resistivity Rw = 100% Sw = 100% Increasing Resistivity (1) Rock (2) Gas (3) Oil (4) Fresh Water (5) Salt Water Conductivity