Petrophysical Challenges in Pre-Salt Carbonate Rocks Requiring Sympathy, Synergy, and Synthesis

Abstract

Reservoir quality in Pre-salt carbonate rocks from the South Atlantic is of major importance. These deposits, largely discovered in the last decade, are characterised by several petrophysical challenges, as might be expected with any carbonate reservoir, but also compounded by relatively little industry experience with non-marine, essentially lacustrine, carbonate facies. Drawing largely on outcrop analogue materials and the limited publications, we have investigated several key issues which we wish to emphasise here:

Understanding of how pore topology effects the porosity-permeability relationships, rock typing schemes and resistivity pathways. We illustrate this in series of plug-scale measurements of coquinas with combination of resistivity index measurements, micro-CT and numerical pore scale models. Resistivity anisotropy is documented.

The role of representative volume scale in the rocks that demands consideration of what scale/volume should be measured and associated upscaling challenges as these volumes change. This can be illustrated by consideration of both coquina and shrub facies examples incorporating outcrop scale models and running flow simulation to calculate effective metre-scale properties.

How variability of rock types is present in single beds over the m-scale, as illustrated by NMR measurements on coquina plug data and assessing the relative contributions of macro, meso, microporosity – micro-porosity appears to be a minor component of these rocks.

Evidence for metre-scale lateral variability of pre-salt coquina facies and modelling the impact at the inter-well scale

Comparison of effective well test permeability with permeabilities averaged from core plug data (across various pre-salt carbonate facies) and from various models for permeability prediction from logs quantifies the importance of ‘missing’ permeabilities in statistically under-sampled (by plugs), highly heterogeneous, rocks.

Several studies have been conducted over the last few years by the authors and co-workers leading to insights that are relevant to the understanding the variation of reservoir properties in these highly heterogeneous systems. We emphasise the need for sympathy with the petrophysical challenges, and synergy between different measurements (i.e., scale of measurements) and subsurface disciplines and the need for careful synthesis of all geological and engineering data as it is possible and cost-effective to obtain in order to predict reservoir performance.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018