Karst feature in Mishra reservoir and effect on drilling and production in Zubair oil field, southern Iraq

Abstract EN

Karst features in the upper part of Mishrif carbonate reservoir can commonly createsubstantial vugs, pores and fissures and, thus, increasing the porosity and enhancing reservoirconnectivity and permeability, this features were formed normally as a result of the carbonaterocks dissolution under the action of meteoric waters during very long periods of emersionspanning from Late Cenomanian to Turonian (about 4.5 Million years).

This study deals withthe presence of the karst features in Zubair oil field, southern Iraq, where Karst features wereobserved in the center and northern parts of the field close to the top of Mishrif Formation inmost of the cored wells.

Characterization of that feature can be achieved from static data atwellbore scale from core, and logs (conventional and non-conventional), and dynamic datasuch as mud losses, well tests combined with production logs (PLT) in addition to Seismicdata.

Beside the positive and economic effect of Karst, there is negative effect which includescausing mud losses during drilling in Mishrif Formation because of low reservoir porepressure, Therefore, acid soluble cement plugs were used to heal that losses to continuedrilling.