This work encompasses redevelopment of a supergiant southern Iraqi oil field from fully vertical to primarily horizontal wells. The subject reservoir is a massive world-class carbonate limestone reservoir containing 23 °API oil and is the dominant reservoir in a set of vertically stacked reservoirs. This reservoir is a part of a large anticline which is oriented north-northwest. The formation is Middle Cretaceous and has a subsea depth of 2100–2600 m. The reservoir was initially developed on a 200 acre inverted 9-spot pattern, and was on primary oil production until reservoir pressure dropped and production declined. Later, water injection started at centers of the 9-spot pattern. Development drilling was projected with future addition of vertical infills at 100 and then 50 acre patterns.
During pressure drop due to primary production, water encroachment from flanks occurred, particularly, in the thin super-high permeability (vuggy) layers present in the reservoir; however, this was not clearly evident at early stages. By the time of starting pattern water injection with comingled injection and production in vertical wells, there was clear evidence of rapid water movement in the vuggy layers. As water injection progressed, the severity of watercut evolution in vertical wells rapidly progressed, necessitating change of the depletion plan. Continued development of the field with vertical infills would result in unsustainable water production and injection requirements, and lower oil recovery.
A redevelopment effort was initiated to overcome these challenges. Geologic and simulation models were modified to reflect the evolved understanding of geology. Appropriate distribution of high permeability layers was introduced and calibrated to production data, in particular, water breakthrough timing and watercut evolution. The main change in the redevelopment plan has been to shift emphasis from drilling vertical infill wells to a fewer horizontal wells targeting low permeability zones which comprise most of the reserves. Reservoir development of the massive lower section is planned with 2 km long horizontal wells with injectors located in lower part of the reservoir and producers higher arranged in a line drive. Upper reservoir development is planned by working over existing comingled vertical wells to upper reservoir only thus decoupling upper and lower comingled production.
The redevelopment plan achieves the business target rate with a much longer plateau duration at lower cost. The plan effectively utilizes existing vertical wells in addition to drilling new horizontal wells for recovering oil from lower, tighter reservoir. Initial performance of horizontal wells have shown very promising results with boosted dry oil production. This updated development plan is now in full execution phase and can provide redevelopment ideas for other brown fields with similar issues.
