Ashraf, Qasim (Weatherford International Ltd) | Khalid, Ali (Weatherford International Ltd) | Ali, Farhad (Weatherford International Ltd) | Luqman, Khurram (Weatherford International Ltd) | Mousa, Ayoub (Weatherford International Ltd) | Babar, Zaheer Uddin (Pakistan Petroleum Limited) | Hussam Uddin, Muhammad (Pakistan Petroleum Limited) | Ullah, Safi (Pakistan Petroleum Limited)
An operator has drilled more than 32 wells to date in Adhi field, a gas and condensate field in northern Pakistan. The majority of these wells produce from depleted sands and some also produce from limestone reservoirs. The wells range in depth between 8,366 and 11,483 ft (2,550 and 3,500 m).
The operator was in the process of drilling the 8 1/2-in. hole section with the least possible mud weight to minimize the overbalance across the lost-circulation-prone limestone formation. While drilling the section, an unexpected gas pocket was encountered and subsequently required an increase in mud weight. To further add to already challenging drilling conditions, a fault was expected in the middle of the section. This fault was expected to produce total losses. The resulting loss of hydrostatic head would have caused a troublesome well-control scenario.
The above conditions led to an inherently tight drilling window. The operator thus made precise management of wellbore pressures a prime objective. However in conventional drilling, relying on the mud weight and pumping rate for accurate management of wellbore pressures proves highly inefficient, if not impossible.
A managed pressure drilling (MPD) and underbalanced drilling (UBD) hybridized system was devised to enable drilling the 8 1/2-in. hole section. An MPD system that applies constant bottom hole pressure would enable drilling the section with the least possible mud weight and as close as possible to the pore pressure line. In the event that heavy to total losses were encountered because of the predicted fault, the system could be switched over to UBD flow drilling. By switching over to UBD, the equivalent circulating density (ECD) would be reduced further and allow the well to flow while drilling and mitigating losses.
An MPD and UBD system was also expected to offer numerous benefits in drilling, including reduced chances of differential sticking, reduced formation damage, increased rate of penetration and bit life, less washouts in the drillstring and pumps, reduced nonproductive time, and enhanced abilities to execute well control with the pipe in motion without fear of getting stuck.
The MPD and UBD hybrid system was deployed to the location. The operator was able to drill the 8 1/2- in. section to the target depth. The operator commenced drilling with an MPD system but, as expected, heavy losses were encountered. Drilling then proceeded with UB flow drilling until reaching target depth. The hybrid system enabled the operator to achieve target depth, eliminate an entire casing string, and substantially reduce NPT. This paper discusses the planning, design, and execution of the MPD and UBD hybrid system.
Zakir, Mufaddal Murtaza (Pakistan Oilfields Limited) | Hussain, Saddam (Pakistan Oilfields Limited) | Qazi, Shoukat Elahi (Pakistan Oilfields Limited) | Ali, Shahzad (Pakistan Oilfields Limited) | Muhammad, Danish (Pakistan Oilfields Limited) | Haleemuddin, H. M. (Pakistan Oilfields Limited) | Hassan, Mohsin (Pakistan Oilfields Limited) | Iqbal, Muneez (Pakistan Oilfields Limited)
Uncontrolled water production in naturally fractured tight carbonate reservoirs with low porosities is a serious problem that confronts Petroleum engineers in Potwar region of Pakistan. The potential of hydrocarbons in these reservoir rocks is masked by the undesirable high water cut. Spectral Noise Logging, High-Precision Temperature Logging in combination with conventional production logging can yield valuable information on gas, oil and water flow profile and behind casing communication in wells.
This paper demonstrates how the (SNL-HPT) diagnostic tool was employed in a well producing from a naturally fractured carbonate reservoir to analyze flow stream to identify hydrocarbon and water flowing intervals and cross flows behind casing. Spectral Noise Logging (SNL) can provide information on reservoir flow units behind one or multiple barriers, which is beyond the spinner capability of PLT. The acoustic noise caused by moving fluid is the result of internal friction and normally audible in highly turbulent flows. Its spectrum and volume is strongly dependent on the fluid type, pressure, temperature and flow rate.
Well-A was drilled and perforated in Sakessar formation of Eocene age. The well flowed at low oil rate and high water cut (98%) despite being at an up dip location compared to a near-by producing well (Well-B) with less water cut from the same zone. To identify the problem, diagnostic logs (SNL-HPT) were run in combination in Well-A to ascertain the perforation’s flow profile and flow behind casing if any.
The diagnostic logs interpretation indicated that the bottom most perforation sets were producing the major volume of fluid and no cross flow behind casing was detected. Based on the analyses of logs, the bottom perforation sets were plugged with calcium carbonate chips capped by cement plug. After water shut off job, the well was flowed and reduced water cut along with decline in gross fluid was observed indicating that the top perforations may have not been acidized. A small acid job was subsequently carried out to stimulate the upper perforation which improved oil rates and the water cut reduced to 60%. Later PLT was also conducted in order to acquire post water shut-off profile and verify SNL-HPT results.