Reservoir Model

The inclusion of reservoir definitions adds improved capacity within Enersight to support conventional exploration and appraisal workflows via an alternative decline input method. Rather than the typical definition of all variables at a well level, this new method enables a calculation on a mass balance basis via defining both well and source reservoir parameters. Initial models’ types include Dry Gas and Black Oil which are both undertaken as a multi-straw evaluation of reservoir on a per-timestep basis to determine reservoir pressure and subsequently well potential. The image below depicts the Reservoir Tab Input for Black Oil type Reservoir:

Click image to expand or minimize. 

General

Input primarily occurs in two places, with each parameter being equipped with individual tooltips and guidance on eligible range of outcome:

  • A new ‘Reservoir’ tab whereby individual reservoirs are defined with a combination of Reservoir and Fluid properties
  • The ‘Production’ tab or ‘Decline/Prod’ input within the ‘Network’ tab where the Reservoir is linked within a new decline method along with individual well properties of pressure, flow coefficient and deliverability exponent.

As this is designed for exploration and appraisal, there is no support for actuals, however prior cum inputs are available to account for exploration or appraisal wells’ production.

Dry Gas

The dry gas reservoir is a multi-straw, single product mass balance simulation, determining the deliverability of gas for the selected wells. Key methods and equations utilized within the calculation process include:

  • Rawlins - Schellhardt equation for the determination of gas deliverability
  • Wichert - Aziz correction factor for determination of pseudocritical properties due to acid gases (H2S and CO2)
  • Hall - Yarborough method for computing gas compressibility factor from pseudo reduced pressure and temperature

Note:

  • The gas composition as defined at the Reservoir inputs contributes to the calculation and overrides individual well composition input values.
  • When dry gas type is selected, non-applicable non-function inputs are greyed out. Function enabled inputs of Water Injection Rate, Gas Injection Rate and Water Ratio are however not relevant to this calculation and inputs to these fields are ignored

The image below depicts the Dry Gas Reservoir type input screen:

Click image to expand or minimize. 

Black Oil

The black oil reservoir is a multi-straw, multi-product mass balance simulation, determining the deliverability of oil, gas and water for the selected wells. As a part of the simulation a material balance is undertaken, iterating to solve using Secant and Newton Raphson Methods.

Workflows associated with this reservoir includes the simulation of:

  • Water Injection
  • Gas Injection
  • Gas Cap
  • Water Flux

Key methods and equations utilized within the calculation process include:

  • Vasquez and Beggs correlation equations for the solution gas oil ratio, oil formation volume factor and oil compressibility as related to saturated / under-saturated status
  • DeGhetto et al. correlation equations for oil compressibility for under-saturated status related to light to extra heavy oil.
  • McCain’s correlation for water formation volume factor
  • Tarner’s Method for predicting Gas Oil ratio (GOR) as a function of reservoir pressure based upon the simultaneous solution of the material balance equation and the instantaneous solution gas/oil ratio equation
  • Fetkovich analytical model is used to estimate water influx
  • Rawlins - Schellhardt equation for the determination of oil deliverability
  • Wichert - Aziz correction factor for determination of pseudocritical properties due to acid gases (H2S and CO2)

Note:

  • When solved per daystep may have slightly different outcomes to monthstep due to the reservoir characterization equations and differing granularity in steps
  • Utilizes the Oil Quality input for determination of original oil in place and the gas composition to avoid doing Sutton correlations for deriving pseudo-critical pressure and temperature, overriding individual well composition input values
  • Reservoir has support for Saturated and under-Saturated conditions as well as compensating for Heavy, Medium or Light oil types
  • Due to the methods selected, the oil composition in the formation was assumed to be fixed over time

The image below depicts the Decline / Prod input for Black Oil Reservoir showing multi-product input:

Click image to expand or minimize.