Applications Of Mwd Technology In Nonconventional Wells, Prudhoe Bay, North Slope Alaska

As production from the Prudhoe Bay Field declines, greater emphasis is being placed upon nonconventional drilling as one method !of increasing production rates and reserves. Identification of lithologic and fluid boundaries through the utilization of real-time and recorded measurement-while-drilling (MWD) technology enables optimum placement and completion of high-angle, horizontal, and inverted high-angle wells. The use of MWD formation evaluation techniques has reduced nonconventional logging costs and allowed acquisition of data where adverse open-hole conditions precluded the use of drillpipe-conveyed logging systems. Real-time gamma-ray and resistivity data are employed while drilling to continually adjust build rates leading to precise stratigraphic placement of the nonconventional wellbore. Recorded measurements, including neutron and density, are made both while drilling (MWD) and after drilling (MAD) to identify gas, light oil, heavy oil/tar, and water. Case studies of a number of selected wells are used to illustrate techniques and approaches used to identify lithologic and fluid boundaries. Overlay interpretation techniques that utilize resistivity and neutron-profile models to account for changes in invasion between MWD and MAD runs are successfully used to identify fluid contacts. Repeat MAD logs are employed when sensors are placed far above the bit as is frequently required by the bottom hole assembly (BHA) designs typically used in nonconventional drilling. Invasion profiles are quantified using neutron, resistivity, and formation exposure time data from tandem tool runs in the Ivishak Formation. Conventional models describing invasion profiles do not adequately explain all the resistivity and neutron profile changes seen in Prudhoe Bay nonconventional wells. In particular, gravity effects on oil-based mud create changes in neutron porosity through time that are opposite to those normally seen or expected in the gas. Nonconventional drilling creates a dynamic interpretation environment influenced by variations in hole angle, mud type, drilling rate, sensor placement, and formation exposure time. With flexible and innovative interpretation techniques, current MWD technology can provide information essential to accurate well placement and can also offer formation evaluation opportunities unavailable from traditional wireline approaches.