A recent opinion in the Santa Barbara News-Press (March 7) argues that there is “no way to tell if drilling reduces pollution.” I disagree, based on nine years of published federally funded research on fault properties and fluid pathways in the offshore Santa Barbara Channel.
In 1982, seep capture tents were installed on the sea floor, about one mile from Platform Holly. This area was selected because it has the most prolific seepage, and gas capture here would have a high impact on air quality. These tents are perhaps the only place in the world that hydrocarbon seep rates can be continuously documented on decadal and longer time scales.
Seep capture reached a peak of about 1600 MCF/day in the late 1980s (MCF = 1000 cubic feet of gas at standard pressure and temperature). Since that time, seepage has gradually decreased to present-day values of about 250 MCF/day.
This is a five-fold decrease in seepage rate. Over the same time period, the decline curve in gas production at Platform Holly has been nearly two-fold. The overall tent seepage decline rate lags behind the platform decline by about a year.
Detailed analysis of seepage into the tent over the past 10 years shows a direct link between new wells completed in the reservoir in close proximity to the tents, but approximately 3,000 feet beneath the sea bed. When these wells are shut off at the platform, natural seepage at the tent increases within days. For example, a one-month shutdown of a well resulted in a 15 percent increase in tent seepage. When the well is opened, the seepage rate returns to the previous value within a few days. This relationship occurs multiple times in the record for that well.
Claims that reinjection through enhanced oil recovery measures may increase pressure resulting in an increased seep emissions are not supported by long-term measurements at Platform Holly. In fact, it appears that the opposite is occurring. Reservoir pressure at Platform Holly has been consistently low at about 1300 psi since the early 1970s. Recently, it has dropped to as low as 900 psi, in spite of reinjection of produced water. These pressures are about 30 percent to 40 percent lower than expected at reservoir depth of about 3,000 feet subsea, where the equilibrium fluid pressure for water-saturated sediment should be about 1500 psi.
How is this possible? Basically, fluids are being removed faster than the surrounding sediment can replenish the reservoir. Our geochemical studies of reservoir water, which has a different composition than sea water, indicate that in some places sea water is being drawn into the reservoir along permeable fault pathways. This is nature’s attempt to maintain equilibrium.
In spite of this process, seep rates have declined around the platform due to production of hydrocarbons, even with reinjection of produced water. Ocean tidal pressure signals are also recorded at reservoir depth in one well adjacent to the fault pathways, indicating communication with the sea bed. The downward movement of fluid along the faults into the reservoir serves to retard the escape of the buoyant hydrocarbons to the sea bed.
In addition, published sonar studies of seepage into the water column by UCSB researchers (1995) show that the most abundant seeps occur in State Lease blocks PRC 308 and 309 (due south of the area between Coal Oil Point and Campus Point), out of the reach of existing platform wells. An earlier study (1973) detected abundant seepage around Platform Holly, but this seepage largely disappeared by 1995.
These trends, considered together, would be a remarkable coincidence if there was not a correlation between hydrocarbon production and seepage. It should come as no surprise that rapid production of hydrocarbons at a rate exceeding their natural replacement (the ultimate reason for the looming fossil fuel problem) results in a decrease in natural seepage rate.
James Boles, professor emeritus
UCSB Department of Earth Science