Wettability and Saturation in Abu Dhabi Carbonate Reservoirs
Authors: Marzouk I. 
Published: December 2007

Wettability of carbonate rocks is a subject of increasing interest in the oil industry in general and in Abu Dhabi in particular. Wettability measurements were carried out for the two major oil reservoirs in Abu Dhabi, namely: Thamama and Arab reservoirs. They represent a wide range of carbonate rocks covering several types of limestones and dolomites. Study of the available wettability measurements has revealed the following observations:

Measured wettability shows a remarkable trend when plotted versus depth, indicating oil wet character in the oil zone, mixed wet in the transition zone and water wet in the water zone. Plotting wettability versus Swi for different rock types indicate oil wet character at low Swi, mixed wet at intermediate Swi and water wet at high Swi. Fresh core resistivity measurements showed variable saturation exponent "n" with changing Swi of the same sample. Cleaning oil wet samples change their wettability to water wet. Aging the cleaned samples with oil change their wettability to oil wet.

All these observations have encouraged us to study, the relation between wettability and saturation and the evolution of rock wettability throughout the geologic history.

After marine sedimentation of carbonate rocks, initial wettability was strong water wet for all the pore types. During oil migration, oil replace formation water in a drainage process. Replacement of formation water by oil started by buoyancy, then followed by thin oil column and an oil/water interface. The differential pressure between oil and water at the interface is the main driving mechanism against gravity and capillary forces. The higher the ΔP the smaller pores invaded with oil.

Oil/grain surface interaction in oil filled pores will last for several million years which will change their wettability from water to oil wet, whereas pore spaces unfilled with oil remain as they were water wet.

Depending on the magnitude of ΔP (Po-Pw), the thicker the oil column thickness, the higher the ΔP, the smaller the pore spaces that will be filled with oil and accordingly change their wettability to oil wet. This may explain the gradual change of wettability by depth from top of the oil zone to the water zone, similar to Sw change with depth. On the other hand, wettability change from one rock type to the other, at the same level afwl, due to variable pore size which reflects Swi.

Therefore, wettability in carbonate rocks is a function of rock type (pore size) and oil column thickness i.e. hafwl. These two parameters are exactly the main parameters controlling Sw in carbonate reservoirs.

Plotting wettability versus Sw indicate that high oil saturations are usually associated with strong oil wet character, intermediate So is associated with mixed wet character and low So is associated with strong water wet character.

When oil enters the pore spaces of carbonate rocks, it changes both saturation and wettability. Measured saturation represent the average saturation % of all the pore types. Similarly, measure wettability represent the distribution of oil and water at the grain surfaces which is the same distribution of oil and water in the pore spaces at the initial conditions. In view of the above, measured saturation at the initial condition represent wettability of carbonate rocks throughout the production/ injection history.