Evening Talk

Evening Talk - "Finding Conductive Clays In A Hydrocarbon Reservoir"

Joanne Tudge, Mike Lovell, Sarah Davies, Iain McNeil, Mike Millar

Thursday 21st April 2008 at Burlington House.

 

Petrophysics traditionally uses Archie's equations to estimate the amount of hydrocarbons in place. This relies on the resistivity response in rock formations being predominantly influenced by the presence of saline pore fluids in the pore space, since the rock matrix is usually non-conductive. Thus the presence of hydrocarbons in a reservoir is usually identified through the electrical resistivity logs because hydrocarbons in the pore space will exhibit a much higher resistivity than water. However, clay minerals present within a sandstone reservoir can provide an extra conduit through which current can travel; this effect is often related to the cation exchange capacity (CEC). If clay minerals are sufficiently abundant in a reservoir, a high CEC can lower the resistivity to such an extent as to compensate for the increase in resistivity caused by the presence of hydrocarbons. In these circumstances there may be no discernable change in the resistivity between the water-bearing and the hydrocarbon-bearing sections of the reservoir. In this so-called "low resistivity pay" situation this error propagates through the petrophysical analysis and results in an overestimation of the water saturations, such that some parts of the reservoir appear to be fully water-saturated yet may flow dry oil.

One approach to understanding this overestimation is to determine the distribution of clay minerals within the reservoir, since their distribution may not be uniform throughout the hydrocarbon bearing reservoir. The formation of conductive clays can be environment dependent, and understanding the processes of how they form can help identify where they might occur within the reservoir.

Comparison of a detailed core-based sedimentological facies scheme that is supported by petrological observations, with wireline log data has demonstrated a link between sedimentological facies and groupings based on wireline petrophysical characteristics such as neutron and density responses. Thus there is a clear link between mineralogical variations in the sandstones and changes in the wireline characteristics. This provides a framework against which to consider the full petrophysical dataset, including measurements on core such as capillary pressure. A link between the occurrence of the clays and different saturation height functions provides a basis towards defining a predictive capability, such that the hydrocarbons in clay-bearing successions are no longer lost.

This study demonstrates how integrating sedimentological and petrophysical data, from both core and downhole data, can have a significant impact towards improving hydrocarbon saturation estimates based on the analysis of wireline logs.