What replaces extracted crude oil?

Answer 1

Simple answer

Within the earth, fluids accumulate in the ground in tiny holes -- called pore spaces -- within the matrix of a rock in a geologic rock formation. When these millions of tiny holes in a rock matrix are interconnected, fluids, such as oil, gas and water, will flow through the rock from tiny pore space to tiny pore space.

Once the oil and/or gas flows out of the interconnected pore spaces and into the well bore, oil and/or gas that is trapped further away from the well bore also begin to flow toward the well bore refilling those pore spaces. Many times water fills in the spaces behind the moving oil and gas until the well produces only water. Other times, there is nothing to refill the spaces and the oil and or gas expand within the presurized well bore reducing the overall pressure of the well bore. Eventually, the pressure drops to a point where the oil and gas no longer flow.

Long answer

Within the earth, fluids accumulate in the ground in tiny holes -- called pore spaces -- within the matrix of a rock in a geologic rock formation. When these millions of tiny holes in a rock matrix are interconnected, fluids, such as oil, gas and water, will flow through the rock from tiny pore space to tiny pore space.

Oil/gas will flow through different rock formations until they encounter a rock formation that is not permeable (it does not have interconnected pore spaces) the oil/gas become trapped. That rock formation is called a trap rock.

When oil/gas are trapped by that trap rock and can no longer flow, they will accumulate in the rock matrix to create an oil and/or gas formation within the interconnected pore spaces of that rock. It is not an open "space" pond, lake or river trapped underground, but oil and/or gas trapped inside the rock formations tiny interconnected holes.

These formations can be as shallow as a few hundred feet, or even 38,000 feet or deeper. The weight of the rock that lies above the depth of the oil formation, called the overburden, creates pressure on the pore spaces where the oil and/or gas are trapped. Suddenly releasing that pressure in an uncontrolled manner leads to a blowout, such as the recent blowout of the Macondo well in the Gulf of Mexico.

When a well is drilled into an oil and/or gas formation, the company usually carefully controls the pressure of that drilled hole or well bore to contain the pressurized oil and or gas within the oil formation. Once the well is ready to produce oil and/or gas, the company will begin to lower the pressure within the well bore. This serves to attract the oil and/or gas trapped in the higher pressured pore spaces within the oil formation and it flows into the well bore. This is commonly seen in nature as a high pressure weather system tends to flow towards a low pressure area, or as air trapped in an untied balloon releases back into the atmosphere when you let it go.

Once the oil and/or gas flows out of the interconnected pore spaces and into the well bore, oil and/or gas that is trapped further away from the well bore also begins to flow toward the lower pressure well bore refilling those pore spaces.

As the oil formation ages, more and more oil is released from the reservoir and several aging scenarios are possible. Two of the most common include: 1) because there is nothing to replace the oil/gas being produced out of the formation, the pressure within the reservoir slowly drops as more and more oil/gas production occurs, which leads to slower production, eventually stopping the flow to the well bore; or 2) water is often trapped below oil/gas in the reservoir and as more oil/gas are produced out of the reservoir, the water continues to fill in more and more of the pores orginally filled with oil/gas until water reaches the well bore and no oil/gas production is possible.

Even with other undisccussed scenarios, as the well ages and without help from the well operator to enhance that oil/gas pressure, the well is spent and will need to be sealed.