When it comes to tight reservoirs, the Barnett shale is about as tight as it gets.
And it’s not enough just to induce fractures to allow the gas to flow, as Barnett drilling pioneer Mitchell Energy discovered after years of applying gel frac treatments to the rock.
The gas did flow, but the pricey gel applications resulted in only so-so economics.
Mitchell engineers cracked the economic barrier in the late 1990s when they ascertained water fracs performed much the same as the gel fracs while dramatically lowering stimulation costs -- and a play was born.
In fact, the Barnett shale play in the in north-central Texas has become so phenomenal (Figure 1) that many industry participants now predict it ultimately will surpass the Hugoton Field as the largest natural gas field in the United States. Already, it’s kicked out more than one Tcf of production, and there are likely several Tcf of booked reserves, according to consulting geologist and Mitchell Energy alum Kent Bowker.
The play took off with water fracing, but it’s horizontal drilling that has propelled it into a true “boom,” noted Mitchell alum and consulting geologist Dan Steward at Republic Energy, which was one of the play’s earliest operators. (Figure 2)
“Horizontals have just swept through this thing,” AAPG member Steward said. “As of 11-1-05, there were 2,135 permits issued for horizontals in the Barnett, with about 5 percent of these being duplicates because of things like changed locations.
“About half of the horizontal permits are in the core areas (sections of Denton, Wise and Tarrant counties),” Steward noted. “The others are non-core or new core, which some people say Johnson County is.”
A Major Player
Not surprisingly, a goodly number of the horizontal wells being drilled can be attributed to the undisputed kingpin of the Barnett: Devon Energy. Devon established its presence in the play in 2001 upon purchasing Mitchell Energy and its already-impressive holdings in the Barnett.
“Devon is pretty much in the core area of Wise and Denton (counties), and we have a large position in Johnson, Parker and western Wise counties,” said AAPG member Jeff Hall, exploration and exploitation manager of Devon’s central division. “We have about 500,000 acres in the heart of the play.
“We’ve drilled 2,040 wells in the play,” Hall said, “and we just finished our 293rd horizontal well. We have 18 rigs running in the play, and 15 of these are horizontal.”
Mitchell pioneered the horizontal drilling effort when it drilled two wells that succeeded mechanically but not economically, due to lack of completion know-how, according to Steward.
Devon later worked diligently to fine tune the horizontal implementations and drilled the first commercially successful horizontal well in the play in 2002 in Tarrant County.
Nowadays it’s commonplace for operators to use horizontal drilling technology in various reservoir types to achieve greater production via increased reservoir contact afforded by horizontal legs versus vertical wellbores. In the case of the Barnett, there are certain geologic conditions that in essence demand horizontal applications.
“The Barnett is dependent on fracture efficiency,” Steward noted. “There’s disagreement over whether or not it’s dependent on induced or natural fractures, but most will tell you it’s an induced fracture play; being able to induce a significant network of fractures at the wellbore is critical.
“Where you have good containment barriers above and below the Barnett and you frac, it (the stimulation) will stay in the zone,” Steward said. “When you get outside the areas of good containment, the percent of success with verticals is not high, but we’re able to frac in the horizontals and keep a high percentage in the zone, so we have good connectivity to the rock -- that’s critical because if you can’t stimulate the rock, it won’t do anything.”
Horizontal technology offers the advantage of drilling multiple wellbores off a single pad, thus leaving a small footprint and enabling access to targets significantly removed from permitted drilling locations. This is a major plus for the Barnett play given that the city of Fort Worth sits atop a vast quantity of Barnett gas.
“There’s so much gas under Fort Worth,” said Nick Steinsberger, consulting drilling and completions engineer and Mitchell Energy alum. “If you move the city, there’s probably several Ts (Tcfs) of gas there. To access it, you must use horizontals and you need long laterals.”
Some folks view such problematic situations as too much hassle, while others see golden opportunity.
Four Sevens Oil Co. is among the latter.
“Our niche is drilling in these urban environments,” said AAPG member Larry Brogdon, partner and geologist at Fort Worth-based Four Sevens, which has acquired considerable experience drilling underneath subdivisions from remote locations.
“We know how to put subdivisions together, which is a lot of work, manpower, a lot of title work,” Brogdon noted. “But (it’s) a good niche for us because some don’t want to be involved in that.
“We lease each home,” Brogdon said, “and we like to put them together in a co-op, which is like a unit. The subdivision sticks together as one unit, so each owner participates in the unit on a pro rata share.
“Sometimes it seems like I’ve turned into a politician and a right-of-way man more than a geologist,” Brogdon noted in jest. “They didn’t teach all this stuff in Geology 101.”
Four Sevens works up its own prospects and currently holds more than 34,000 acres in the play, with about 22,000 of this in the core area. Three rigs are running, and these folks lay their own pipe because everything they drill is in some kind of municipality where there’s no pipeline infrastructure.
“Some of the best part of the play is right in the urban areas,” Brogdon said, “and some we’ll never get to because there’s too much culture and you can’t get pipeline infrastructure in there.
“Sometimes you can’t find enough places to drill that are far enough away from homes to get a permit from the city to drill,” Brogdon added. “But there are some areas where there are creeks, floodplains from rivers, industrial corridors where we can find pad sites and drill underneath subdivisions, business parks, things like that.
“When we plan pad sites, we’re not only looking for places to drill but looking for a route where we can bring a pipeline, which is key.”
Location, Location, Location
In the Barnett, the preferred drilling orientation is perpendicular to the stress field, according to Brogdon. However, because suitable surface well site locations are something of a rarity in heavily subdivided, densely populated urban areas, the well pad may have to be positioned in a poor location on the primary lease, or even on an adjacent lease.
To achieve proper wellbore orientation in these challenging situations, Barnett operators are employing a new type of horizontal drilling approach dubbed the “turnizontal” where the well kicks off drilling in one direction to reach the target penetration point and then turns to its final azimuth direction toward the bottom hole at the secondary kickoff point.
Steward noted 3-D seismic has been a critical part of the horizontal wellbores. This is because operators are not only trying to avoid structural complexities, but also striving to understand the dips and dip rates in the areas where these wells are being drilled.
“There are a lot of advances being made in what we’re doing with 3-D data,” Hall noted. “As we drill more and look at well performance and go back and look at the seismic, we start to see trends and realize there’s a lot more value to seismic than just avoiding bad areas.”
Looking to the future, Hall said Devon and others will continue to expand the productive limits of the play and establish the remote counties as either productive or not.
Where the Barnett is not productive, the challenge will be to determine what, if any, technology might reverse that.
Given that some politicians in Washington, D.C., are noisily demanding the industry make the nation less reliable on imports while simultaneously refusing to allow drilling in new promising domestic areas such as Alaska’s Arctic National Wildlife Refuge (ANWR) and the Outer Continental Shelf (OCS) outside of the Gulf of Mexico, a play like the Barnett becomes particularly significant.
“What’s remarkable is as an industry we’ve all looked at international projects with big reserves,” Hall said, “and with the advent of the Barnett, as an industry we’re starting to realize these older basins have a lot of hydrocarbons left.
“We just need to think about them differently -- like shales as big time reservoirs,” he noted. “No one thought until the Barnett there would be this kind of opportunity in these shales. It’s really exciting to have a really large gas field in an old productive basin.
“It gives me encouragement there’s a lot of hydrocarbons left to be found and a lot of opportunities and challenges we can undertake in the future.”