The central Luconia gas province located offshore Sarawak houses numerous carbonate reservoirs (Fig. 1). Some of these reservoirs are characterized by the presence of karsts and fractures, contributing to total mud losses. Implementing the variant of managed-pressure-drilling technology called pressurized mud-cap drilling (PMCD) allowed targeted total depths (TDs) to be reached on several wells. However, reaching TD alone is insufficient for conclusive evaluation. Integration of technology applications is paramount in increasing the success rate of data delivery.
Introduction
Well-known for highly varying formation properties even within small sections of reservoir, the recently discovered Malaysian carbonate formations present a risk of drilling-fluid losses. Conventionally, these wells were drilled with overbalanced mud and losses were cured with lost-circulation materials (LCMs) or cement plugs to enable restoration of overbalanced condition for future drilling. Drilling a multiple total-losses zone conventionally proved to be highly uneconomical and likely to jeopardize the safety of the operation. To mitigate this inefficiency and associated safety risks, the PMCD technique was implemented and made ready for all of the operator’s carbonate drilling operations. Since 2010, PMCD equipment and personnel have been mobilized for 11 wells, but have been used only in six wells.
Introduction to PMCD
PMCD is applied in the total-loss condition. Once sufficient loss rates are encountered, the annulus is displaced from overbalanced drill-weight mud to underbalanced light annular mud (LAM). The LAM is generally designed to be underbalanced to the formation pressure at the topmost fracture by approximately 100 psi. This enables gas migration into the wellbore to be monitored closely. After casing pressure has increased to a limit, LAM is injected into the annulus to bullhead the migrated gas back into the formation. Upon each injection, the casing pressure will be restored to the original 100 psi. Drilling operations are performed while injecting seawater continuously down the drillstring and intermittently injecting LAM down the annulus. Drilled cuttings will be carried by seawater and fed into the fractures. Planning for successful PMCD operations not only revolves around equipment and engineering, but also depends on well placement and architecture, well engineering, rig equipment, training, and logistics.
Successful PMCD Implementations in the KUN2 Well
This well was drilled in August 2012 by use of a semisubmersible drilling rig. Upon drilling 11 m into the carbonate section, a total loss of drilling fluids was experienced. Loss rate was recorded at approximately 1,200 bbl/hr. The well was successfully drilled to TD in PMCD mode. Once at TD, the bottomhole assembly (BHA) was stripped out and a composite bridge plug (CBP) was run into the hole and set inside the production casing (i.e., several joints above the shoe). The well was then displaced to kill fluid before the running tool was retrieved to surface. A drill-in liner system was then run into the hole and used to drill the CBP. The well was converted back to PMCD mode once the CBP was drilled out, and the liner was then run to TD. The well was secured upon setting of the liner-top packer.
Main Challenges
The main challenges faced in drilling carbonate exploration wells include determining the top of the carbonate to enable the correct setting depth for production casing, ensuring efficient PMCD operations, acquiring pore-pressure data, isolating the well after drilling to TD in PMCD mode, and determining an optimum well-killing method after performing production testing.
Solutions
Optimization of Carbonate Prediction With Seismic-While-Drilling (SWD) Technology. To ensure successful PMCD operations, a hole section has to be dedicated to the carbonate section. The presence of shale within this section will introduce borehole-stability risk once the drilling operation is converted to PMCD. To prevent shale exposure in the PMCD section, determining the correct section depth for the hole section before carbonate is of great importance.
To accomplish this goal, SWD was used in the KUN2 well. During SWD runs, vertical seismic profiles were initiated during pipe connection and waveforms were transmitted to the surface during drilling operation. The result can be analyzed by the project geologist and geophysicist on site to produce an updated top-of-carbonate depth prognosis while the hole is being drilled. The accuracy of the prognosis will increase as seismic is initiated deeper and closer to the target formations.
Ensuring Efficient PMCD Operations. One of the main challenges in ensuring successful and efficient PMCD operations is drilling in one bit run. This will significantly minimize consumption of LAM and eliminate the exposure to well-control risk when the BHA has to be tripped out of hole under a total-loss condition in the middle of the operation. From past experience, the main problem hindering this goal is LWD failure caused by vibration and drill-bit wear. While the latter definitely requires a bit trip, LWD failure during PMCD drilling also mandates a trip out because LWD is the only viable means of obtaining openhole formation evaluation once total losses are encountered.
Acquisition of Pore-Pressure Data in Total-Loss Condition. Because of the inability to perform openhole wireline logging after converting to PMCD mode, the operator deployed a formation-pressure-while-drilling (FPWD) tool in PMCD/total-loss mode. The challenge of obtaining a good pressure point in PMCD mode derives from the absence of mudcake (with seawater being used as drilling fluid) and from the potential existence of fracture lines across the borehole wall. In KUN2, borehole imaging while drilling was included in the BHA to facilitate the selection of a good pressure point. Throughout the reservoir section, 25 pressure points were taken, with the majority failing to achieve good seals. When the few valid points were compared with data obtained from the production test, it was found that the measurements from FPWD were inconsistent with the production-test data. Because there is higher confidence in the reliability of data from production testing, it was concluded that the FPWD data were questionable, which could be attributed to the abnormal and unconventional wellbore conditions observed in PMCD mode.
Because of the challenges involved in obtaining pore-pressure data when the well is converted into PMCD mode, an alternative measurement method is proposed in the KUN2 well using annular-pressure-while-drilling (APWD) measurements. In theory, the use of pressure-while-drilling capability will enable estimations of pore pressure in the reservoir section in PMCD mode. The idea is to measure equivalent-static-density data from the APWD tool during connections, when there is no LAM being pumped into the hole. It is assumed that, when left static for a sufficient time, the fluid in the borehole will gravity segregate, with an LAM column fully supported by the pressure at the topmost fracture.
Isolations of Wellbore Against Total Losses. From past experience, repeated attempts to prevent losses with concentrated LCM pills of up to 120 lbm/bbl will be unsuccessful, the key reason being the nature of the loss zones themselves (i.e., cavernous/vugular). Cement plugs were tried but demonstrated a poor success rate, mostly achieving success only upon the seventh attempt or even later. Because of the failure of these methods, the gunk-plug option was explored. Gunk plugs, in theory, seemed to have ideal characteristics, providing temporary isolation while not plugging the vugs. Though the first pilot test of gunk plugs in early 2012 was largely unsuccessful, gunk plugs seemed to be a likely solution.
In addition, CBPs were used as an isolation solution. CBPs are made of substantially nonmetallic components, usually composed of a fiber-and-resin mixture such as fiberglass or of high-performance plastics. Because of the nature of the composite materials, the CBP is drilled out easily in a single-pass operation. After the CBP is set, the well will be displaced to kill fluid to overbalance the pressure below the plug once it has fully flipped to gas. Previously, the operator had twice used the CBP successfully as a temporary isolation method for carbonate wells. In KUN2, the CBP was not set after five attempts. On the last attempt, the string was turned and reciprocated as an attempt to clean the casing wall, which eventually allowed the CBP to be set successfully. (See Fig. 2 for the CBP-running sequence.)
A third solution, a drill-in liner-hanger system, was used in KUN2 in combination with a premium high-torque gas-tight connection and six-bladed drillshoes. In KUN2, a CBP was drilled with LAM pumped through the drillstring. After approximately 15 minutes of drilling, a total-loss condition occurred as the CBP was drilled through and PMCD mode was activated again. Fluid injection was switched to seawater, and the liner was reamed down to TD. After TD, the liner was set hydraulically. Because of a persistent total-loss state, achieving good cementation was almost impossible. To ensure well integrity for testing, the operator had practiced installing three floats for the liner and a tieback packer to provide seal redundancy at the liner lap.
Well Killing After Production Testing. This process is a major challenge. Experience has shown that even pumping a massive volume of LCMs will not cure the losses, so it has been agreed that no attempt will be made to cure the losses after a production-test operation. The wellbore will only be isolated from the losses zone with a mechanical device. To date, the operator had implemented this strategy successfully in several wells where an expandable bridge plug had been run through the test string and set above the perforation zone. Cement plugs were then set above the bridge plug with several dump bailer runs and allowed to harden before being pressure tested. The well was then displaced to kill fluid to allow retrieval of the testing string.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164576, “Integrated-Technology Approach To Explore Carbonate Reservoirs in Malaysia Enhances PMCD Potential and Enables Successful Prospect Evaluations,” by M. Noreffendy Jayah, SPE, Intan Azian A. Aziz, SPE, Zulhilmi Drus, SPE, Thanavathy Patma Nesan, Wong Han Sze, SPE, Abdel Aziz Ali Hassan, and Pungut Luntar, Petronas, prepared for the 2013 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, San Antonio, Texas, USA, 17–18 April. The paper has not been peer reviewed.