STUDI GEOMEKANIKA 3D DENGAN ANALISIS BIDANG LEMAH LAPISAN (PLANE OF WEAKNES) DI LAPANGAN ALPHA, MADURA UTARA

Abstract

Alpha field which located at offshore North Madura, East Java Indonesia, is known for the economic hydrocarbon reserve based on the previous drilling campaign results. The success of the production results had strengthened the decision to further develop this field by targeting several upcoming wells. But drilling process in this field was not easy and encounter many drilling problems such as breakouts, stuck pipes, well collapse and even wells need to be sidetracked. From the drilling perspective, the carbonate dominant formation is challenging and shows high uncertainty for the drilling parameter. 1D geomechanics analysis was applied in the previous drilling campaign to provide the analysis of rock properties and recommended drilling parameters. The wellbore instability problem has been reduced after the 1D geomechanics analysis application, although the main drilling problem still occurred such as breakout and well collapse. Blocky caving then observed in some of the wells in the interbedded carbonate and shale in Kujung Formation, and this type of caving is potentially indicating the plane of weakness issue. Based on those drilling experiences, the advanced geomechanics study is mandatory for the safety and successful well drilling.

 Anisotropy in the rock properties over the study field may lead to the invalid 1D geomechanics model in the prediction of rock properties for the upcoming well trajectory. Besides that, the pressure and rock stresses are also varying in every location where it is affected by tectonics and fault plane. Based on that, the disadvantages of the 1D geomechanics model need to be escalated to the 3D model with support from the seismic attributes and 3D static reservoir model. 1D logs of rock properties in every offset well were upscaled and propagated over the field as a 3D cube. 3D cube of rock properties with pore pressure and vertical stress then will be the inputs to calculate 3D horizontal stresses over the field by using Blanton & Olson Formula.

 The presence of the blocky caving in several wells in the Kujung Formation indicates another factor that can cause well collapse. However, this factor was not analyzed in the previous drilling campaign. This blocky caving potentially represents the bedding plane failure due to the low attacking angle of the well trajectory and formation bedding. To avoid bedding failure, the stresses applied to the bedding plane due to pressure from drilling activities must be calculated by using Kirsch Formula. The stress applied to the bedding plane can be maintained by using correct mud weight and reduce the well inclination.

 3D geomechanics model helps in the rock properties prediction in the trajectory of the upcoming well. Added to that, the plane of weakness analysis helps to predict the stress applied in the bedding plane due to drilling activities. The integration of the 3D geomechanics model with the plane of weakness analysis provides the accurate drilling parameter and the mud weight recommendation for safe and successful well drilling and increased the cost efficiencies.