Underbalanced Techniques to Reduce Drilling Hazards of Loss of Circulation and Underground Flows

Abstract

Abstract This paper discusses underbalanced drilling planning, implementation and risk management to solve a drilling hazard where a weak upper formation is open to a lower high pressure gas reservoir that has pore pressure greater than the frac gradient of the weak formation. The problem has been solved in the Burgos Basin where loss of circulation and underground blowouts are a major hazard for drilling operations. The paper presents the planning process, risk assessment/HAZOP analysis, computer modeling with a steady state 2-phase flow model, equipment specifications, operational guidelines and field results from an underbalanced operation undertaken in August 2003 to successfully drill a well where there was a continual inflow while a weak lost circulation zone was exposed. Description and Results This paper documents a well drilled in the Burgos Basin where loss of circulation and underground blowouts are a major hazard. The well had suffered an underground blowout and a sidetrack was planned to recover the original hole and to reach the programmed well objectives. Risk assessments were performed which identified mitigations needed to minimize and control risk. Equipment design specification and installation were executed as per operational design for UBD. The operations maintained stable downhole pressure equilibrium of no losses or underground flows and the well successfully reached the original planned TD and completion requirements. Shown are the operational, risk assessment and field control guidelines used. The use of risk assessment, 2-phase flow calculations and operational guides are profiled for the operation. Lastly lessons learned are presented for safe and improved performance. Applications The principle idea was to use UBD techniques to solve difficult situations where the frac gradient of the open hole interval would not support a kill weight mud (e.g., the situation where kill weight mud looses circulation and lighter mud weight allows kicks and will flow underground if the well is shut in). In this situation UBD techniques saved the cost and complications of running additional contingency strings. Description of the Problem The Burgos basin is one of 4 major gas basins in Mexico. It is located on the western side of the Gulf of Mexico in the Mexican states of Tamaulipas and Nuevo Leon covering an area of 50,000 km2 and has about 1000 producing wells from 181 fields. The basin was discovered in 1945 and currently produces approximately 1.0 BSCF/D of non associated gas. The well where these techniques were employed was part of the Burgos Project in the La Cuenca de Burgos region. The reservoir is located in the north eastern part of Mexico, 60 kms from the Texas border. The geology is fairly consistent throughout the field and the Burgos Basin is considered a High pressure, High temperature environment located in Paleocene and Eocene formations. Since the formations are fairly young and naturally fractured there are many drilling hazards. Some of the unconsolidated intercalated weak sands are especially problematic when several gas bearing sands are crossed in the same hole section where losses and kicks occur simultaneously and there is no mud weight that will solve either problem successfully.