5.7: Summary and Final Tasks

Summary

In this lesson, we discussed three very important tasks performed by reservoir engineers working on natural gas reservoirs. These tasks are identical to those performed by reservoir engineers working on oil reservoirs. These are:

• estimation of the Original-Gas-In-Place or OGIP;
• estimation of the stabilized production rate from a vertical gas well; and
• forecasting the future performance of gas reservoir and wells.

We saw that there were two methods commonly used in the oil and gas industry estimating the OGIP:

• the Volumetric Method
• the Material Balance Method

We also discussed that of these two methods, the material balance method is typically assumed to be more accurate as it is based on dynamic data.

We discussed the four Drive Mechanisms associated with gas reservoirs:

• gas expansion (most significant drive mechanism in conventional gas reservoirs)
• gas desorption (may only be present in certain unconventional gas reservoirs)
• rock and fluid expansion (expansion of the reservoir rock and interstitial water – typically only significant in over-pressured gas reservoirs)
• natural aquifer drive (or water encroachment)

We saw that the gas desorption mechanism is only applicable for unconventional gas reservoirs with significant organic content in the rock. The criteria of high organic content in the rock is normally met in:

• coal seam methane reservoirs
• shale gas reservoirs

We also discussed the stabilized production rates from vertical gas production wells. We saw that there were several flow regimes possible in a hydrocarbon reservoir (both natural gas and crude oil) that occurred at different stages in a well’s productive life. In Figure 4.06 from Lesson 4, we saw that these flow regimes occurred sequentially as the pressure disturbance caused by production propagates outward from the well towards the boundaries. These flow regimes are:

• Well dominated flow
  • Wellbore storage
  • Damage/Stimulation
• Transient flow
• Late transient flow
• Boundary dominated flow
  • Steady-state
  • Pseudo steady-state

In both gas and oil wells, the initial well dominated flow regime is controlled by well properties and well damage/stimulation, not reservoir properties. The transient flow regime is governed by the diffusivity equation and is characterized by time dependent rates and/or pressures. The onset of the late transient period occurs when the pressure disturbance from the production well reaches the first boundary of the drainage volume. The end of the late transient period occurs when the pressure disturbance reaches the last boundary of the drainage volume. The boundary dominated flow regime is the last flow regime experienced by the well. Stabilized production occurs during the boundary dominated flow period and is described by one of the Inflow Performance Relationships, IPRs that we derived in this lesson.

In natural gas wells, three different formulations can be used to develop the stabilized IPRs and the transient diffusivity equation. These formulations are based on the manner in which the term, $\frac{p}{{\mu }_{g}Z}$, is integrated in the development of the different equations. These three formulations are:

• Pressure Formulation: valid for $p>3,000psi$
• Pressure-Squared Formulation: valid for $p<2,000psi$
• Pseudo-Pressure Formulation: valid for all pressures

The Pseudo-Pressure Formulation is the most rigorous formulation within any pressure range of interest (even if another formulation is valid in that pressure range).

For forecasting future reservoir or well performance, we discussed two methods: Material Balance Analysis and Decline Curve Analysis. In the Material Balance Method, we needed to modify the governing equations from their oil counterparts to account for the highly compressible nature of gas. We did this by incorporating the Real Gas Law into the equations rather than the definition of a slightly compressible fluid (note, water and its expansion in a gas reservoir are still treated as slightly compressible). For Decline Curve Analysis, we did not need to make any modifications to the Arps^[2] Equations.

Final Tasks