When a well on a map is clicked, a production curve (Rate vs. Time, log x linear axes) is displayed. Below the production curve is the Decline Curve Analysis section.
Background on Decline Curve Analysis:
Essentially, decline curve analysis is a method for forecasting future production for certain types of oil and gas wells. The shape of a well’s decline curve is primarily determined by the well’s permeability and its reservoir drive mechanism:
Moderate to high permeability, depletion drive oil and gas wells exhibit a constant decline rate after some period of time. Constant decline rate is also known as exponential decline. When the production curve from a well exhibiting constant decline rate behavior is plotted on a log x linear chart, the curve will approximate a straight line. The slope of this line is the annual decline rate, and is typically represented with units of %/year. Thus, future production from these types of wells can be estimated using decline curve analysis for exponentially declining wells.
Lower permeability, or “tight” wells, tend to exhibit what is known as hyperbolic decline rate behavior. Specifically, the decline rate in these wells is proportional to the production rate itself, so the rate tends to decrease with time. Later in their life, the rate of change of the decline rate for hyperbolically declining wells tends to decrease. At this point, the future production of these wells can be approximated using decline curve analysis for exponentially declining wells.
Water drive wells can show little or no decline until “the water hits” and the well “waters out”. Production from water drive wells should not be forecast using decline curves.
OMSYS’s Decline Curve Analysis tool is for use with constant decline rate, or exponentially declining wells. As discussed above, it can also be used on hyperbolically declining wells where the rate of change of their decline rate has declined sufficiently.
How to use the Decline Curve Analysis tool:
Study the production curve (Rate vs. Time, log x linear axes) and determine if linear (straight-line) behavior is present. Remember, if there is little or no decline, a well may be too new to be exhibiting decline or may have a water drive reservoir. If so, the Decline Curve Analysis tool cannot be used.
If linear behavior is present, click on the Begin Point Selection button.
Move the cursor to a point on the major phase curve (the major phase of an oil well is the oil curve, the major phase of a gas well is the gas curve) where the linear behavior begins – i.e., the start of the straight-line decline behavior. As you move the cursor, note that the month/year and production rate are displayed in a hover box. Click when you locate the point you desire. If you wish to change your point after clicking, just click on Begin Point Selection and the form will be cleared.
Move the cursor to another, more recent, point on the linear portion of the major phase curve. Click and the point will be selected, and the decline will be calculated between the two points. Once again, if you wish to change one or both of the points, click on Begin Point Selection and the form will be cleared. (Note: it is not possible to change just one point – both points must be clicked each time the Begin Point Selection button is clicked, with the oldest being clicked first.)