Abstract
One of the main causes involved in reservoir formation damages is the excessive mud filtrate during drilling with an overbalanced mud weight. Results show reservoir pressure retreat due to geological structural failure can leads to a large disparity between MW and PP especially in HPHT environment.
This new approach focuses on the geological structural failure that causes the well bore trajectory to cut through different transgressive and regressive pressure envelops. If the mud program, based on the pre-drill pore pressure model, did not account for a pressure retreat, a sizeable lost circulation can take place. Mud fluid invasion to the open borehole wall can decrease permeability, impact logs measurements and potential production. Moreover, thick mud cake causes bore-hole tight spots, excessive torques and pulls. Incorporating the subsurface geological structure features to the seismic driven pre-drilling models can foresee the unexpected mud weight overbalance intervals.
In relative old sediments, such as Oligocene and Miocene in the offshore shelf areas, a substantial mud weight (MW) increase of 2-5 ponds is needed to penetrate the top of geopressure (TOG). The deeper geopressured compartments are usually drilled with MW that sometimes reaches 18 pounds per gallon. Moreover, temperature can range from 300 to 350 degrees Fahrenheit. Formation damages and serious challenges take place where the MW is overbalance juxtaposing a regressive permeable rock sequence. Study the relationship between geopressure compartmentalization and drilling challenges of multiple wells in East – West Cameron, offshore Louisiana sheds light on one of the important causes of formation damages, tight spots, high torque and pulls. Pay zones were partially masked due to deep mud filtrate invasion with frequent use of oil-base to reduce pulls in the tight spots. Sidetracks usually was the ultimate remedy for some of the wells that did not reach their objective TD. The case histories shown in this study illustrate that exploring the depth to the possible regression zones before drilling in conjunction with the pre-drill pore pressure model could have alerted drillers for MW amendment to avoid lost circulation.
The benefit of correlating the bore-hole trajectory to the seismic semblance associated with the geopressure profile is a resourceful method of foreseeing the troublesome intervals. In real time, calibrating the MW derived from the seismic pre-drill model is another deterrent process to avoid utilizing excessive overbalanced MW.
