Infill drilling optimization is a crucial aspect of the economic development of oil and natural gas fields. When an oil or gas reservoir is discovered, the operator needs to plan on how best to recover as much of the original gas or oil in place within the reservoir. Knowledge about the reservoir is gleaned from the wellbore, and the reservoir’s extent is based on prior geological and geophysical information until other wells are drilled to prove the lateral extent of the reservoir. At ground level, the aerial extent of the reservoir is commonly known as a field.

Any gas or oil reservoir will have a given amount of “storage space” known as porosity (i.e., the volume within rock that can contain fluids). Gas or oil occupying this storage space is under pressure, and generally, the deeper the location of the reservoir, the higher its pressure. A reservoir’s pressure is the energy utilized to push the gas or oil to the wellbore. Once a well punctures the reservoir, this stored energy will decline, and the efficient use of this energy helps to assure maximum recovery of hydrocarbons. Many reservoirs will have sufficient pressure to push the gas or oil to the surface on initial discovery, hence a flowing well. The measure of a rock’s ability to transmit fluids—in this case, to the wellbore—is called permeability.

It will take too many years for a single well to cost-effectively drain a reservoir. Drilling additional wells is necessary for efficient and optimal recovery of gas and oil from any reservoir. There is an optimal distance between each well such that they will not interfere with each other’s ability to produce efficiently. Known as well spacing, this distance is usually expressed in acres.

In the Cotton Valley Sands gas reservoir of the East Texas Basin, the development wells were drilled originally on 640 acre spacing (equal to 1 square mile). More than 30 years after the discovery well was drilled, the well spacing is down to 20 or 40 acres, depending on operator and area of operations. The production gained from this infill drilling could not have been realized from wells on a 640 acre or 320 acre spacing. The Cotton Valley Sands is a “tight” (low-permeability) gas sand formation, and flow to the wellbore is restricted. A technique known as hydraulic fracturing is applied to the reservoir to increase the permeability and the rate at which the gas can flow to the wellbore. This technique is costly, so it becomes imperative for Cotton Valley operators to optimize their infill drilling efforts.

At some point, a reservoir’s energy will be unable to move the gas or oil to the surface. From this point on, pumps or lifting mechanisms will be installed in the well to provide artificial lift. Beyond this level of effort for many oil reservoirs, the reservoir energy can be increased by injecting a material in a well that will push the oil to another well. Water is the most common material used in such an effort, but some operators inject steam, CO2, or chemicals. Sometimes the existing well pattern will be utilized in this injection program; in other cases, optimal operations will require that additional wells be drilled for efficient injection schemes and the subsequent recovery of gas or oil.

NETL supports research into the nature and aerial extent of reservoirs, detection of additional reservoir space, and mapping the internal architecture of a reservoir. NETL research into infill drilling optimizatin also extends to technology that will reduce the cost of drilling wells as well as the ability to drill several wells from a single location.