Goals

• Reservoir Management - Schaben Field
• study the effect on well productivities
  • stimulation
  • well pump rates
• infill drilling program - well placement

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Volumetric Study

Prior to the start of simulation, on a grid-by-grid basis, determine if reservoir parameters (effective porosity and net pay thickness - water saturation) support historical production.

Establish grid pattern that properly defines the reservoir by using any mapping or gridding software (Geographix used to generate the grids for this study)

• Size Of Grid Cells In X & Y Direction
• numerical dispersion reduced - 4 to 5 grid cells between wells
• grid sizes of 220 ft by 220 ft - 5 grid blocks between wells

• reservoir heterogeneity
  • schaben field is a dual porosity - dual permeability system
  • oil column migrated across multiple zones
  • most cores contain vertical fractures
  • mini-permeameter results - high & low permeability layers
  • production history - initial water free production followed by rapid onset of water production
  • current reservoir pressure near original reservoir pressure - suggests strong recharge by aquifer
• multiple completions in isolated reservoirs
• segregate openhole & casedhole completions by layer

Simulation designed as two layer model - a reservoir layer with an underlying aquifer layer. Z-dimension equals pay thickness (top of the reservoir to the OWC, average depth of OWC = -2145 ft subsea)

Layer 1 - Reservoir (gross/net pay thicknesses assumed to be equal)

• difficult to distinguish between productive and non-productive zones
• porosity and permeability averaged over pay thickness

Layer 2 - aquifer provides recharge to reservoir layer (aquifer thickness in Schaben Field approximately 100 feet)

• Pay thickness - grid dimension in z direction
• Porosity
  • dual porosity system
  • total porosity - from petrophysical logs (Pfeffer)
  • total porosity = effective porosity + ineffective porosity
  • ineffective porosity saturated with water
  • effective porosity saturated with oil, gas, & water
  • fluid flow occurs only through effective porosity
  • NMR studies on core plugs at reservoir pressure
    • correlates effective porosity to total porosity
  • core porosity assumed equal to log porosity
• fluid saturations
  • dichotomy - Sw from petrophysical logs & production history
  • average Sw - 65 to 75%.
  • production history - high OWR in early years followed by water break through
  • thus all of Sw is not mobile
• effective saturation calculations on grid cell basis
  • OOIP = (1-Sw) * grid volume * total porosity
  • effective pore volume = grid volume * effective porosity
  • effective So = OOIP / (effective pore volume)
  • effective So - normalized <= 75% - relative permeability curves indicate Swi = 25%
• effective Sw = 1- (effective So) (water saturation in effective porosity)
• Map of OOIP on a grid cell basis - effective porosity * effective So * grid volume
• Map of cumulative production
  • Available production history - 33 years lease production history & well productivity tests.
  • Calculate well production - annual productivity tests - distribute lease production to wells. Bo (=1.04) - surface volume to reservoir volume.
  • Schaben Field - uniform 40 acre spacing with 36 grid cells per acre.
• Cumulative production per well divided by 36 (grid cells) to allocate production over drainage area to generate cumulative production per grid cell map.
• Map of ROIP per grid cell
  • ROIP = OOIP – cumulative production
  • remaining So = ROIP / effective pore volume
  • remaining So thickness = remaining So * pay thickness
  • Soir = 25% (assumption)
  • residual mobile So = remaining So - Soir - So => 0 - reservoir parameters consistent with production history
  • Sor * t = residual mobile So * pay thickness - buried treasure map - areas with significant production potential
• Map of Recovery Factor - on a grid cell basis – cumulative production / OOIP