NEW ORLEANS – BP’s effort to drill a relief well through 2½ miles of rock to stop the Gulf spill is on target for completion by mid-August, the oil giant said Friday. But BP’s stock tumbled anyway over the mounting costs of the disaster and the company’s inability to plug the leak sooner.

The relief well is considered the best hope of halting the crude that has been gushing since April 20 in the biggest offshore oil spill in U.S. history.

The crew that has been drilling the relief well since early May ran a test to confirm it is on the right path, using a tool that detects the magnetic field around the casing of the original, blown-out well.

“The layman’s translation is, ‘We are where we thought we were,’” BP spokesman Bill Salvin said.

Several such tests are necessary, because drilling sideways into the original well casing requires boring through more than 13,000 feet of rock to hit a target 9 inches in diameter, or about the size of a dinner plate.

Once the new well intersects the ruptured one, BP plans to pump heavy drilling mud in to stop the oil flow and plug it with cement.

Despite the encouraging news, BP stock fell 6 percent in New York on Friday to a 14-year low on news that BP has now spent $2.35 billion dealing with the disaster.

BP has lost more than $100 billion in market value since its deep-water drilling platform blew up, and its stock is worth less than half the $60 or so it was selling for on the day of the explosion.

BP is drilling two relief wells, in case the first one misses its mark. The first one, started May 2, reached a depth of 16,275 feet Wednesday — including about 5,000 feet of water — before workers paused for the test.

The relief well is only 200 feet laterally from the original well, but the crew still must drill about 2,000 feet deeper before it can intercept the original well, according to Salvin. The second relief well, started May 16, has reached a depth of 10,500 feet.

The biggest oil spill ever in the Gulf of Mexico — an undersea gusher in Mexico that started in the summer of 1979 — was eventually stopped with two relief wells. some estimates, the BP spill could eclipse that disaster in a week or two; the spill has been put at somewhere between 69 million and 132 million gallons.

BP is capturing anywhere from 840,000 to 1.2 million gallons of oil a day. Worst-case government estimates say 2.5 million gallons a day are leaking from the well, although no one really knows for sure.