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Providing coverage of Alaska and northern Canada's oil and gas industry
February 2006

Vol. 11, No. 7 Week of February 12, 2006

Gravity tracks Prudhoe gas cap waterflood

Gravity meters coupled with precision GPS receivers measure minute gravity changes from waterflood in the Prudhoe gas cap

Alan Bailey

Petroleum News

Petroleum engineers have been tracking the progress of the waterflood in the Prudhoe Bay gas cap under Alaska’s North Slope by measuring surface gravity changes that occur as water replaces gas in the reservoir more than 8,000 feet underground.

BP Exploration (Alaska) petroleum engineer Jerry Brady told a Geophysical Society of Alaska audience on Feb. 2 how this state-of-the-art reservoir surveillance technique requires global positioning system receivers that can measure ground elevations to within 2 centimeters and gravity meters that take gravity readings to within a few microgals. A microgal is a gravitational acceleration of one millionth of a centimeter per second squared — roughly equivalent to a house fly landing on the back of a 20-ton whale, Brady said.

The history of the waterflood monitoring project goes back to a decision several years ago to use waterflood to maintain reservoir pressure in the Prudhoe Bay gas cap. Petroleum engineers faced the issue of how to monitor the movement of the injected water underground without drilling observation wells.

“One of the questions was how do we adequately monitor this waterflood, given that there are very few wells in the gas cap at Prudhoe,” Brady said.

Gravity measurement has solved the problem.

Precision gravity meters

A key technology in this solution is the modern gravity meter, used to take gravity measurements at about 200 observation stations on a surface grid above the Prudhoe Bay reservoir. There are two types of meter in use at Prudhoe Bay: one type measures the relative weight of a test mass between different locations and the other type measures the absolute gravity at a location.

The tool of choice has become the absolute gravity meter, Brady said. This type of meter measures the gravitational acceleration by dropping a crystal inside an evacuated chamber. It works like the timing of Newton’s famous apple, except that laser-based movement sensors and an atomic clock give the device a precision that might have astounded the discoverer of the laws of gravity.

The meters can read to 3 to 5 microgals or less in ideal conditions. But on the North Slope background noise levels in the readings come in somewhat higher than that.

“In a survey that we’re doing on the slope right now we’re getting about plus or minus 10 microgals of noise level,” Brady said.

GPS

A precision global positioning system receiver determines the exact position of a gravity meter at an observation station during a gravity measurement. A technician makes repeated GPS measurements at a station, to ensure that the accuracy is within required limits.

“By using differential GPS we can usually get our elevation to, hopefully, within about a centimeter,” Brady said. Measurements exhibiting errors of more than about 2 centimeters are rejected, he said.

Accurate elevation measurement is critical because it impacts the gravity value. A 1-centimeter elevation error equates to about a 3-microgal gravity error, Brady explained.

However, North Slope surface movement caused by heaves in the permafrost and varying ice conditions can play havoc with precision terrain measurements. The engineers working at Prudhoe Bay have addressed this issue by installing four permanent GPS stations.

“Everything moves a little bit from time to time,” Brady said. “… So when you’re talking about getting something that’s a centimeter, you’ve got to do something a little bit different.”

The fixed GPS stations, with theoretical accuracies of about 5 millimeters, record positional information continuously. Then, during a survey, triangulation of observation stations back to the fixed stations enables checking of the integrity of the mobile GPS stations used for the observations.

Baseline and repeat surveys

Prior to the start of waterflood, two baseline surveys provided gravity readings when gas still filled the gas cap.

“We actually performed two baseline surveys … which really worked out well, because by having two baselines it gave us a good idea of what our noise level was,” Brady said.

After the start of the waterflood operation, repeat surveys, typically carried out each winter, measure changes in gravity as the injected water moves radially though the reservoir from seven central injector wells. The replacement of gas by water in the reservoir causes an increase in reservoir density that results in an increase in gravity, or positive gravity anomaly, at the surface.

“We’re simply taking the difference between a repeat survey and the baseline survey, and from that we get the gravity anomaly,” Brady said.

Calculating the density

Calculations performed on the pattern of gravity anomalies measured at the surface during the repeat surveys enable the calculation of the pattern of reservoir densities. And that calculated density pattern tells engineers the extent of the waterflood in the reservoir, Brady explained.

A computer simulation enables an assessment of the accuracy of the method. That simulation shows that the gravity anomalies accurately determine the area of an average amount of water front fill. However, the gravity anomalies place the leading edge of the waterflood about 2,000 feet too far out from the center of the waterflood area.

“So if you want to know where the bulk of that water is we can nail that pretty tight,” Brady said. “The leading edge — we’re going to be off a couple of thousand feet or more, probably on that.”

It is also possible to verify the accuracy of the method by performing a mass balance on the amount of water injected. The 4 billion barrels of water to be injected into the gas cap during the entire multi-year waterflood project should result in an eventual gravity anomaly of about 250 microgals. To date, the anomaly is up to 80 or 90 microgals, a value consistent with the amount of water injected so far, Brady said.

“So it’s fitting in really close to close to what we predicted,” he said.

Borehole gravity surveys

Brady is also intrigued with the use of borehole gravity meters for gravity surveys within well bores.

A borehole gravity meter, especially in a horizontal well, could monitor fluid drainage around the well without being impacted by well liners and other well features. And a gravity meter also ought to provide greater accuracy than logging techniques currently used — the gravity meter should particularly enable the determination of the area of drainage into a well.

“When you look at horizontal wells really, to me, it would revolutionize how we do our surveillance,” Brady said.

And Brady thinks that the permanent installation of borehole gravity meters in wells offers particularly strong potential for reservoir surveillance, once suitable instruments for this type of metering become available. There are currently a couple of patents out for permanent borehole gravity sensor designs, he said. One of these designs consists of an absolute gravity meter in which a piezo-electric device flips a crystal in a vacuum chamber, he said.

Other applications

However, success with the surface gravity surveys at Prudhoe Bay shows that that particular technique could find applications elsewhere. The relatively large density contrast between water and gas makes the technique work well with waterflood in gas — waterflood in oil would be more challenging because of the much lower density contrast between the two fluids involved.

The depth and thickness of the reservoir are also critical parameters for success. The Prudhoe reservoir is 100 to 500 feet thick, but modeling of the technique suggests that it could work reliably with reservoirs down to about 50 feet thick — 25 feet would be the absolute minimum thickness.

“It’s certainly not for every field, but there are lots of fields out there where it would work,” Brady said.






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