Factbox: How a relief well works

(Reuters) - BP Plc is drilling a pair of relief wells intended to kill the gushing leak in the Gulf of Mexico, which could happen in August at the earliest. Until then, the company is ramping up stopgap efforts to contain as much crude as possible from its Macondo well.

Here is an explanation of how a relief well works, as explained by industry and academic experts as well as Kent Wells, BP's senior vice president of exploration and production.

* A relief well provides access to a blown-out well far beneath the seabed, at or close to the bottom of the problem well.

* Typically, it is drilled parallel to the problem well through multiple layers of rock and sometimes salt. Then the drillbit curves to intersect with the problem well. This is how BP's relief wells are being drilled.

* The first relief well began drilling on May 2, and the second began on May 16.

* As of June 18, the first well had been drilled to 10,677 feet, or 2 miles, beneath the seabed. The second well had reached 4,662 feet, or eight-tenths of a mile.

* The first well also was within 200 feet of the side of the blown-out well, but had to continue drilling down to find the right intersect point.

* The Macondo well was drilled to 13,000 feet, or 2.4 miles, beneath the seabed.

* The drilling process is lengthy because it must stop at points along the way. That allows drillers to insert piping, called casing, to hold the well open and prevent a cave-in.

* The diameter of the well shrinks as it drills deeper to maintain control and integrity of the wellbore. BP's target will be about 8.5 inches across, or about the size of a large dinner plate.

* BP is slowing the pace of drilling for the first relief well so electromagnetic sensors can be used to detect the blown-out well and gradually move closer.

* BP has detailed information on the Macondo well that is helping it choose the right path for the relief wells.

* BP intends to first pierce the space between the wellbore and the casing in the blown-out well, and then pierce the casing. That will ensure the well connects with the flow path of gushing oil and gas.

* Once intersected, BP can pump heavy drilling fluid down the relief well into the blown-out well.

* The weight of the mud reduces high pressures in the reservoir that send crude billowing up to the leak.

* As pressure is reduced, the flow of oil slows.

* Once sufficiently slowed, BP can pump cement into the Macondo wellbore and plug the leak at or near the source.

* A relief well can still work if it doesn't precisely intersect with a blown-out well.

* In that case, if a relief well gets close enough, holes can be punched through its casing to allow drilling mud to flow through fissures and fractures in the reservoir rock to reach the blown-out well.

* Once the leak is plugged, BP can possibly return to the Macondo well at some point to try to produce oil from it. BP Chief Executive Tony Hayward told Congress the company estimated the reservoir holds up to 50 million barrels of oil, and the well would have produced 15,000 to 25,000 barrels a day had it been completed.

* If the Macondo well is too damaged to revisit, it is possible the company could turn one of the relief wells into a producing well.

(Reporting by Kristen Hays; Editing by Xavier Briand)