A True Shale Compaction Model With Pore Pressure Prediction
Authors: Krushin Jim T 
Published: May 2013

Currently, the industry relies on shale velocity and electrical resistivity trends to model compaction and pore pressure. These methods do not properly identify the interrelated shale compaction properties, important to geoscientists, as well as drillers, of porosity, pore pressure, and cation exchange capacity (CEC, meq/g). CEC is a measurement of the amount of exchangeable charge per unit mass of dry sample. The specific components of compaction are mechanical, governed by effective stress; thermal, associated with increasing temperature due to the geothermal gradient, and chemical, such as the smectite to illite transformation. In addition to the components of compaction, the variables of CEC (meq/g) and the specific exchangeable cation (Na is believed to be the most dominate species in the subsurface) also affect shale porosity. Desorption water vapor isotherms of Na-exchanged pure clays, clay mixtures, and shales help to quantify the variables and components that control shale porosity. The isotherms are simply a measurement of the gravimetric water content, (WC, water-g/dry sample-g) as the relative humidity (p/po) is decreased from 100%. Water vapor desorption isotherms show that the WC for a given (p/po) increases with increasing CEC. Plotting p/po vs. WC normalized by CEC (mass of water-g/meq) results in the data collapsing to a single general trend. With knowledge of the bulk CEC of the shale, a proto-mechanical compaction curve results from this trend when a simple thermodynamic equation converts p/po to effective overburden stress. Additionally, water vapor sorption isotherms measurements at different temperatures calibrate the thermal compaction component associated with the geothermal gradient. The effects of diagenesis (i.e. smectite to illite) and variable shale mineralogy are incorporated in the compaction model via the bulk CEC parameter, calculated from petrophysical well logs using a refined published technique. After accounting for temperature and CEC, the shale porosity, derived from the bulk density well-log, is dependent only on the effective overburden stress state, i.e., pore pressure. There is excellent agreement between the modeled shale pore pressure outlined here with pore pressure inferred from drilled mud weights and pressures measured in interbedded sands. Since, shale is so abundant and challenging to drill, a robust compaction model will help explorers and drillers by properly modeling seismic, forecasting geologic technical risk, decrease well costs, and improve drilling safety.

Introduction

Sedimentary basins typically contain about 70% shale (Potter et al., 1980; Jones and Wang, 1981). Since shale is so abundant in the strata drilled for hydrocarbons, quantifying shale properties helps with predrill prospect evaluation via geologic and seismic interpretation, basin modeling, and wellbore design. Additionally, since shale causes over 90% of wellbore stability problems (Talabani et al., 1993), knowing their properties can help decrease drilling well costs and improve safety. In this report, the term " shale?? is used as general classifications of very fine grained clastic rocks that loses porosity through compaction and regains porosity through swelling. Though, many geologists and engineers refer to montmorillonite as the " swelling clay??, all clays swell as do nonorganic shales (Chenevert, 1970; Low, 1985; Molinda, et al., 2005). As discussed below, the swelling strain, the increase of porosity with decreasing effective stress (and technically decrease in temperature, though this is not seen as a realistic geologic concern in a passive margin basin like the Gulf Coast) is proportional to the cation exchange capacity (CEC, meq/g) of the bulk shale. The goal of this paper is identify and illustrate the interrelationship among the shale properties that control compaction and ultimately to infer pore pressure derived from porosity. Since pore pressures cannot be measured directly in the ultralow permeable shales, indirect methods of are used to evaluate the compaction modeled in this paper. These indirect methods include pore pressures measured in interbedded sandstones (MDT's) as well as drilling mud weights, deemed to be correct.