Strict Control of ECD in DW Exploratory Well Using a 2nd Generation Continuous Circulation System

Abstract

Abstract In an industry more increasingly focusing on efficiency and cost reduction, controlling well pressures during drilling is a fundamental requirement for operational efficiency, economic success and personnel safety, especially with very narrow margins and more extreme deepwater, high-pressure/high-temperature (HP-HT) environments. The use of continuous circulation system (CCS) as a simple managed pressure drilling (MPD) solution or as complementary technology to backpressure-control MPD in our industry focusing on reduced well costs and drilling time should be more diffused for its numerous benefits. It provides a downhole steady-state condition and, as a result, the formations do not suffer from pressure oscillations while hole cleaning and borehole stability improve, reducing the chance of problems in open hole. The practice of Continuous Circulation Drilling in a deep-water exploratory well, solves many drilling challenges through a continuous dynamic condition in the well and consequent benefits in terms of constant equivalent circulating density (ECD) and bottom hole pressure (BHP), continuous hole cleaning and solids transport. All these advantages result in significant safety standard gains, with reduced likelihood of kicks and losses during connection times, mitigation of stuck pipe incidents and improvements of rate of penetration (ROP). The first CCS was a ‘Coupler’. The "development of the ‘Coupler’ became the subject of an ITF (Industry Technology Facilitator) Joint Industry Project in October 2000, supported by five major oil companies."1 After its first field protype land application in 2003, starting from 2005 many operators began using the ‘Coupler’ in tight pore pressure and fracture pressure windows. The needs to easily integrate and manage the CCS in every rig in operation moved the industry to a sub based configuration with lower footprint on the rig operations. Low systems reliability, potential safety issues and uncertain connection times slowed down the establishment of continuous mud circulation as a routine application in the drilling industry, even though this technology represents a substantial drilling breakthrough. For these reasons, a 2nd generation CCS has been developed to make continuous circulation drilling a standard drilling practice in any land and offshore applications suffering drilling issues related to bottom hole pressure management, poor hole cleaning and borehole instability, with consequent low ROP and costly non productive time (NPT). This 2nd generation is able to perform connections at full circulation rate with high safety standards and performance. At the heart of the evolution were an innovative sub lateral port configuration with a double pressure barrier configuration, an automated clamp to open and close the sub lateral port through a PLC controlled procedure. "Exploratory wells are in principle classified as challenging ones due to the inherent uncertainty relative to the downhole pressure limits, pore and fracture pressures, as well as formation surprises that are not clearly mapped from seismic. In addition, the exploration frontier is moving increasingly towards more difficult environments, and it is no surprise that many exploratory wells have been abandoned before reaching the planned total depth (TD), therefore failing to meet its commercial or technical objectives of collecting valuable data about possible reservoirs and hydrocarbon reserves."2 The paper describes the operations and results obtained by the 2nd generation CCS and its benefits in terms of cost control and safety enhancements in a deep-water exploratory well in Black Sea with a calculated pore pressure – fracture gradient (PP-FG) profile with a high level of uncertainty due lack of calibration wells and seismic velocity anomalies due to gas effect. The major Oil Company utilizing this system recorded a close control of the equivalent circulating density (ECD) during connections and improved borehole cleaning. Potential kicks during connections have been avoided and the system performed 58 operational days with 100% success rate, zero accidents and zero LTI.