Abstract
Given the mature phase that the industry is in, companies can no longer afford a haphazard approach to operational risk. Risk must be quantified to assure that a proper return is being paid for it. And one drilling operational risk that must be quantified is differential sticking owing to the loss that this problem has cost the industry over the years. The loss is ever increasing as the industry is forced to access depleted reservoir formations that expose her greatly to the differential sticking problem in her bid to increase hydrocarbon recovery.
This study seeks to quantify (give a sense of magnitude) to the risk of being differentially stuck in drilling a depleted reservoir formation. This is project seeks to identify drilling parameters that contribute to differential sticking which can be engineered to provide drilling conditions that will always allow a high probability (P90) of being able to pull free in case of differential sticking
A deterministic model is built first then a stochastic model is obtained from the deterministic model to incorporate the uncertainties surrounding most of the modeled variables. The stochastic model is built and run with risk analysis software @RISK. Sensitivity analysis is carried on the key variables contributing to the sticking force to ascertain the degree to which each one has contributed to the resultant sticking force. The modeled variables include: BHA and Drillstring Design, Mud Design and Mud Cake Morphology, Formation Characteristics and Drag and wellbore geometry. The model was tested by applying it to 3 wells drilled onshore Niger Delta.
The built model both deterministic and stochastic predicted accurately the occurrence of severe differential sticking in well #1 and 2 and also predicted minimal differential sticking in well #3 and this is conformity with the field experiences in drilling these wells. The sensitivity analysis showed that the variables affecting differential sticking are mud cake thickness, length of embedded pipe mud weight and sticking coefficient.
It is of paramount importance to quantify the differential sticking risk of drilling a depleted reservoir formation. The model developed can with a good degree of accuracy quantify this risk and through sensitivity analysis identify variables that should be adjusted to reduce this risk to an acceptable limit of (P10).
