Predicting rock mechanical properties of carbonates from wireline logs (A case study: Arab-D reservoir, Ghawar field, Saudi Arabia)

Four hundred plug samples from the Arab-D carbonate reservoir, Ghawar field, were tested for acoustic and mechanical properties at increasing triaxial stress. The results show that the rock mechanical parameters are primarily functions of porosity and, to a lesser degree, of mineralogy, texture and pore fabric (in order of degree of impact from higher to lower). The rock mechanical parameters of the intact matrix rock show no significant changes with stress under reservoir condition.The study enabled the generation of general and layer-specific correlation formulae of porosity with P-wave velocity, S-wave velocity, static and dynamic constants and the angle of internal friction. The formulae were then applied to a key well in the Ghawar using the compressional wave slowness from a Multipole Array Acoustic Log (MAC™, Baker Atlas) to derive rock mechanical pseudo-logs on reservoir-scale (referred to here as general) and on individual reservoir zone scale (referred to here as layer-specific). Comparison made between the general and the layer-specific pseudo-logs shows good agreement for each of the elastic constants with matching peaks and troughs throughout the logs. In addition the laboratory derived rock mechanical constants show a good agreement with the pseudo-logs. Where there is some difference between the general and layer-specific pseudo-logs, the layer-specific log shows better correlation with the laboratory derived constants. It can be concluded that the porosity correlation is an accurate, representative and cost effective method of obtaining a rock mechanical profile of the Arab-D reservoir. The derived formulae have been implemented as predictive tools in reservoir development and management (e.g. hydrofracturing and underbalanced drilling) and new prospect evaluation. The rock mechanical layering scheme shows higher resolution in the prolific part of the Arab-D reservoir than the Saudi Aramco conventionally used zonation (Each of Zones 2B and 3A consists of two rock mechanical layers). Furthermore the least prolific zones (lower part of 3A, the whole of 3B and 4) form one rock mechanical layer.