Ultrahigh-pressure/high temperature (UHP/HT) wells are very challenging because of the narrow drilling margins, which can cause either losses or an influx. In a field in Malaysia, the pore pressure and fracture gradient were too close to allow conventional drilling, so the managed pressure drilling (MPD) technique was used to drill these wells.
Cementing under MPD is a new technique in this area. A technical assessment revealed that there is limitation in the currently used cementing placement simulator in that it cannot simulate the real-time MPD cementing placement. The current hydraulic simulator does not account for automated real-time annulus choking to apply the backpressure and safely place the cement in the annulus. In addition, it does not take into account the downhole viscosity change due to pressure and temperature effects and which is not imposed by the testing protocols from API. To check the effect of the downhole conditions on the fluid viscosities, which will have an effect on the friction pressure in the annulus during the placement, and to reduce the risk during the cementing job, two in-house hydraulic simulators were identified and used to benchmark the currently used simulator results; the design workflow was changed to account for the additional simulations, and the new technique was implemented successfully to cement under MPD conditions.
The Duyong field was discovered in July 1970 with the drilling of three exploration wells—Duyong-1, Duyong-2, and Duyong-3. In 1976, PETRONAS Carigali took over the field and drilled four appraisal wells in 1982 prior to installing the platforms for development. The first commercial gas from this field was produced in April 1984.
The Duyong Deep-A (Fig. 1) is an ultrahigh pressure/high temperature (UHP/HT) well (Fig.2); it is located 236 km northeast of Kemaman supply base in water depth of 75 m. The primary objective of the proposed Duyong Deep-A is to evaluate the hydrocarbon potential of the group M sandstone at depths of 3200 to 4300 m with an anticipated pressure from 8,000 to 13,000 psi and a temperature from 335° to 450°. The deepest offset well drilled in this field is Duyong -3, which was drilled in 1974 at a depth of 3159 m.
To add to the complexity to the design of this well, the M sands have never been explored before. Therefore, estimating pore pressure was complex. The pore pressure prognosis for the Duyong Deep-A has been carried out by integrating the relevant pressure data from the basin modeling analysis; the pore pressure in the targeted deeper M group sands has been predicted only on the basis of the seismic velocity and basin modeling. In view of the low confidence in the estimate, direct and indirect pore pressure measurements were performed during the drilling to determine the lower bound of the drilling margins. Measurements were obtained through the following:
Geomechanical predrill study and pore pressure modeling
Formation pressure measurement using wireline
Dynamic flowback using managed pressure drilling (MPD)
Below 3000 m true vertical depth (TVD), the pore pressure and the fracture gradient were clearly unknown and were based on best estimates. The variation of the drilling window was estimated to 0.12 SG to 0.24 SG based on the current pore pressure and projected facture gradient (Fig.3).
The uncertainties of the drilling window below 3000 m TVD are huge; thus, rigorous efforts were made to cater for equivalent circulating density (ECD) management during the drilling and cementing. These efforts included
Low-solids mud system for rheology optimization