The Pembina Cardium oilfield is the largest oilfield in Canada, covering 283,000 hectares (700,000 acres) and containing estimated reserves of 1.2 billion cubic metres (8 billion barrels) of original oil in place. Expected recoveries from waterflooding are low, estimated at only 21 to 23 percent of OOIP. Early water breakthrough has caused declining production in many regions of the pool.

Although macroscopic heterogeneities drastically affect waterflood performance, oil recovery must ultimately occur at the pore level. This study is concerned with the examination of sandstone, the major producing component of the Pembina reservoir, for heterogeneities in wettability and pore structure at the microscopic level.

Wettability was determined using imbibition tests and photomicroscopy of samples where both oil and water phases were immobilized. Miscible displacements and mercury porosimetry provided pore structure information. Waterfloods and oilfloods gave additional insight into the effects of both wettability and pore structure.

Non-random channel type heterogeneities were found to exist at the microscopic level. Pembina sandstone contains a broad range of pore throat sizes, with small pore throats controlling large pore volumes. Minerals containing microporosity are non-uniformly distributed. Large variations in permeability were measured within a very small region of the reservoir.

Pembina sandstone was found to have a heterogeneous state of wettability. Bicontinuous mixed wettability was observed, with samples imbibing both brine and oil. Photomicrographs were taken showing the smallest pores to be oil wet, providing visual agreement with electrical double layer calculations found in the literature which predict a minimum pore size for aqueous wetting film stability.

Examination of waterflood and oilflood curves reinforced the concept of mixed wettability. Waterfloods on the mixed wet cores showed many characteristics of waterfloods on oil wet cores, probably due to the continuous oil wet pathways through the cores. Comparison of waterfloods on extracted and unextracted cores showed that wettability plays a major role in causing early water breakthrough. Channel type heterogeneities in pore structure were also shown to cause early water breakthrough in unextracted and extracted cores. It can be concluded that both wettability and pore structure contributes to the early water breakthrough observed in the Pembina Cardium reservoir.