### Summary

In this lesson, we discussed three very important tasks performed by reservoir engineers:

- estimation of the Stock Tank Oil Originally In-Place or STOOIP
- estimation of the stabilized production rate from a vertical oil well
- forecasting the future performance of oil reservoir and wells

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

- the Volumetric Method
- the Material Balance Method

We 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 also discussed the five Drive Mechanisms associated with oil reservoirs:

- Rock and fluid expansion
- Solution gas drive
- Gas cap drive
- Gravity drainage
- Natural aquifer drive (or water encroachment)

Later in the lesson, we saw how these drive mechanisms could be quantified and put into the material balance equation

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

- Well dominated flow
- Wellbore storage
- Damage/Stimulation

- Transient flow
- Late transient flow
- Boundary dominated flow
- Steady-state
- Pseudo steady-state

The 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 regime experienced by the well. It is during this period that stabilized production occurs. Stabilized production is described by one of the Inflow Performance Relationships shown in **Table 4.02**.

For forecasting future reservoir or well performance, we discussed two methods:

- The Material Balance Approach
- Decline Curve Analysis
- Exponential decline
- Hyperbolic decline
- Harmonic decline

The Material Balance Method is an analytical (theoretically based) approach based on measured rock and fluid properties. It accounts for all drive mechanisms encountered in the field. Gravity drainage is considered implicitly in the choice of the datum depth used to evaluate the reservoir pressures. Decline curve analysis is an empirical (observation based) approach that uses trends in the observed data and analysis of these trends using simple mathematic expressions. Decline curve analysis is categorized based on the value of the b-parameter: exponential $\left(b=0\right)$, hyperbolic $\left(0<b<1\right)$, and harmonic $\left(b=1\right)$.

Harmonic decline results in unrealistic EURs and is typically used as the theoretical limit for the b-parameter.

### Final Tasks

### Complete all of the Lesson 4 tasks!

You have reached the end of Lesson 4! Double-check the to-do list on the Lesson 4 *Overview *page to make sure you have completed all of the activities listed there before you begin Lesson 5.