Undulating horizontal drilling techniques of the type successfully used to develop viscous oil on Alaska’s North Slope could prove a key to unlocking BP’s 120 million barrel Badami field. BP restarted production at the eastern North Slope Badami field in September 2005 after a two-year shutdown that resulted from uneconomically low oil production rates.
“A greater reservoir understanding, coupled with high oil prices and new drilling technology, is providing some encouragement that may lead to the implementation of a new well design on a pilot basis and possible redevelopment (of the field),” Bill Bredar, BP’s Badami project geoscientist, told an audience at the joint meeting of the Cordilleran Section of the Geological Society of America, the Pacific Section of the American Society of Petroleum Geologists and the Western Region of the Society of Petroleum Engineers on May 8 in Anchorage, Alaska.
Bredar said that production from the field dropped precipitously after an initial production rate of 17,000 barrels per day at field startup in 1998. The reservoir pressure slowly built up again after the field was shut down, he said. The field is currently producing at about 1,500 bpd from four of the original production wells.
Brookian turbiditesPeople have long known that the Badami reservoir consists of layered sandstones known as turbidites in what geologists term the Brookian sequence, a set of Cretaceous and Tertiary rocks deposited in what was at the time a marine basin called the Colville basin. Turbidites form when mixtures of sand and water flow periodically into a marine basin — each flow typically results in a thin layer of sand that lies sandwiched between layers of fine silt or mud.
People have also long known that the production problems at Badami result from the Badami reservoir consisting of many small individual compartments, rather than a few large sand bodies. The small compartments limit the ease with which oil can flow through the reservoir to the production wells.
But Bredar explained how the drilling and seismic surveying associated with the development and initial production at Badami have enabled a detailed understanding of the field reservoir and how that understanding may lead to successful redevelopment of the field. In fact, the reservoir description for the field changed significantly as the development drilling proceeded, he said.
Base of the basin slopeBredar showed a cross-section of the field in which a seismic image clearly depicts the relics of the Brookian near-shore, shallow water shelf, the deep basin and the slope connecting the shelf to the basin. As is typical of turbidite deposits, the Badami reservoir sands accumulated near the base of the slope in what was at the time deep water.
Geologists have mapped six large-scale systems of sand deposits within Badami, Bredar said. Each system consists of what is known as a submarine fan, in which the turbidite sands have splayed out across the basin floor after pouring through channels down the slope from a shelf area several miles away.
But the fans form the largest scale of five levels of compartment within the Badami reservoir, Bredar explained. Each fan, typically 300 feet thick, consists of two to seven 100-foot thick lobes that can clearly be recognized on seismic sections. Then each lobe consists of a series of channel complexes, each approximately 50 feet thick, 1,200 feet in width and two miles in length (channels within these complexes represent the routes along which the turbidite flows passed).
Seismic data cannot resolve detail below the scale of the fans, lobes and channel complexes. But geologists know from well data that fourth and fifth level compartments, invisible to the seismic data, also exist. The fourth level consists of individual channels within the channel complexes and the fifth level consists of individual turbidite flows within the channels.
And within the reservoir geologist have recognized four sandstone types, with permeabilities ranging from 10 millidarcies in thin-bedded varieties to 100 millidarcies in thicker types. Porosity averages 19 percent, Bredar said.
Original well designWhen BP first developed Badami, people recognized that a hierarchy of reservoir compartments existed and the well designs targeted stacked oil pays in the sandstone layers in the fan lobes, Bredar said.
“The well designs that were implemented in the initial development were a response to the reservoir description at the time,” Bredar said. “… The original Badami prize was supposed to be through access to multiple stacked pays in various fan lobes, which led to vertical and near-vertical well designs.”
The near-vertical wells penetrated layered sand pays, each well drawing oil from multiple layers. The wells used several different completion types, to accommodate variations in the reservoir characteristics. And, to address lateral discontinuities between sand channels, two wells involved trilaterals, with the lateral wells extending downward “like three-legged stools,” Bredar said.
But none of the well designs proved successful.
“Unfortunately regardless of completion and design type each of these vertical wells had a precipitous drop in production,” Bredar said.
Bredar went on to explain how evidence from trilateral wells that targeted channel complexes (the third level of compartment in the reservoir) pointed to the reasons for the production problems.
“The distance between the … laterals is in the hundreds of feet, yet sand body continuity is poor,” he said. This and other examples of sand discontinuity between closely spaced wells “lead us to the conclusion that level-four heterogeneity (caused by individual turbidite channels) is field wide.”
Not entirely sealedAlthough the discontinuities between individual sand bodies presents some significant problems for reservoir engineers, the reservoir pressure and production rates have recovered significantly when the field has been shut in. That ability for the reservoir to recharge gives reason to hope that Badami can produce effectively.
“The reservoir is heavily baffled but does not consist entirely of sealed compartments,” he said.
Bredar described a detailed study of the production from one Badami well. An initial steep decline in production preceded a flattening production rate. The different phases of behavior of the well, coupled with the fact that injection techniques were not used to enhance recovery, indicate that over time the well was making fluid contact with sand bodies increasingly distant from the well bore.
So, bringing well production rates up to economic levels becomes a question of working out how to entice the reservoir to recharge fast enough while a well is producing. And that will involve targeting the fourth and fifth level compartments within the reservoir.
“The Badami drilling challenge is to not only penetrate the multiple nested channel complexes where the thickest and best sands are but … to drain individual channels and individual beds within those complexes,” Bredar said.
And the answer to meeting that challenge seems to be the use of undulating, horizontal multilateral wells that thread their way through the small-scale compartments. The key drilling targets now become the productive sand compartments in the basal lobes of the fan systems “both laterally and to a lesser extent vertically,” Bredar said. The undulating horizontal well design can address both the horizontal and the vertical compartmentalization.
“So as a result of all the geologic and reservoir engineering information collected from Badami’s first development phase, an undulating horizontal well design that can move between compartments in the basal lobes of the Badami reservoir should provide better access to Badami’s oil resources,” Bredar said.
And reservoir modeling of the horizontal well design as an alternative design for one of the original Badami wells has estimated a production improvement of perhaps 2.8 times for that well, Bredar said.
“Badami is a challenging reservoir,” Bredar said, in conclusion. “The integration of geology and engineering has led to a better understanding of the compartmentalization. It’s not the worst case, as recharge (of the reservoir) is proven. The compartments are not completely sealed and new drilling technology is available. Redevelopment will begin with a pilot sidetrack (well).”