Formation Damage Impact on Defining Fluid Contacts

A. Al-Ghamdi¹

Abstract

Defining fluid contacts in wells is a very significant task in understanding fluid distributions across any given reservoir. In one example from Saudi Arabia, the free water level (FWL) is inconsistent across the field. On the west flank, the FWL elevation is higher than the middle of the field by almost 200’. This interpretation is based on well test data where water was recovered. Furthermore, on the east flank of the field, the FWL plunges to the north with an overall 300’ difference in water level. Different hypotheses have been used to explain these anomalies without solid explanation.

This study shows that formation damage has occurred over the reservoir interval for the tested wells where water was recovered on the west flank and FWL was interpreted, indicating that differences in FWL interpretations are due to formation damage, rather than an inherent field-wide characteristic such as a fresh water influx. The investigated wells were drilled with high overbalance mud in an area with low quality carbonate rocks. High overbalance drilling in low quality rock magnifies mud filtration invasion, which can cause pore bridging and pore-throat blocking. Cement and perforation jobs can also aggravate formation damage by adding cement filtration fines and pulverizing a compacted zone near the wellbore. No cleanup practices were used to overcome this damage. Moreover, the new interpretation takes into account the time lag between drilling and production history.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain