PETROLEUM DEVELOPMENT GEOLOGY RESERVES ESTIMATE

• Can be a simple volume calculation or a complex net gas or net oil isopach approach, determined by structure contours modified by fluid contacts and net isopachs net reservoir thickn

• The most important role of a DG is to:

be discovered in a particular venture.

ventures.

1 Educated Guess and/or Comparison

with nearby production.

2 Static Reserves Estimates

Ü Volumetric Calculations

3 Dynamic Reserves Estimates

Ü Decline Curve Analysis

Ü Material balance calculations

Ü Reservoir Simulation

• Consider a region where production is from a highly fractured tight formation or where

poroperm heterogeneity is unpredictable.

• Volumetric calculations are largely

meaningless.

• A way to estimate potential production from

a well is to consider those nearby.

• Generally, such a wildcat well will not

perform better than the nearest wells: best to estimate cautiously

• Most accurate and widely used methods of reserves estimation.

• Carried out by geologists as they are based on

geological structure and isopach maps.

• Rock volumes are established that are assumed to

contain hydrocarbons (e.g seismic bright spot).

• Can be a simple volume calculation or a complex net gas or net oil isopach approach, determined by

structure contours modified by fluid contacts and net isopachs (net reservoir thickness map).

• Accuracy of volumetrics depends on data for porosity, saturation, net thickness, areal extent, formation

volume factor, integrity of those data within a reservoir.

Volumetric Method

• RR = 7758 x A.t x φ(1 – Sw) x FVF x RF

̇ RR = Recoverable Reserves

̇ 7758 = conversion from acreft to barrels (if vol in

m3 this conversion number is eliminated)

̇ A = area of porous rock, acre

̇ t = thickness in feet

̇ (1-Sw) = water saturation of reservoir

̇ FVF = Formation Volume Factor (1/Bo & 1/Bg)

̇ Bo/Bg å reservoir volume / surface volume (vr / vs )

̇ RF = Recovery Factor

• Most rock volumes established through use of net gas and net oil isopachs (net pay map).

• Constructed from superimposing of net isopach map and structure contour maps then cut

(reduced) it with well defined OWC and/or GOC

• Calculate the volume of net pay map by

planimeter (or digitizer table) and/or grid square counting

NET PAY MAP

Rock Volume Calculations

• For oil with effective water drive the

primary recoveries are in 25 – 40 % range (max 75%).

• For gas with gravity drainage, water drive and depletion drive can provide RF > 80%.

Average Oil Recovery

Decline Curve Analysis

(Reservoir Engineer’s jobs)

• After wells have been producing for a while:

– The rate of production is graphed

– Generally 6 months – 1 year after start of production

• Good reserves estimates can be derived

Often compared with volumetric technique results.

• Can be done by well, by a group of well, by block, by reservoir, by field

Decline Analysis Results

• Determine remaining recoverable reserves under natural depletion rate.

• To forecast production under existing

conditions

• Limitation:

– The degree of the accuracy is depend on the reliability of the production data

Production Plots

1 A plot of log(q) vs t is

p Linear if decline is exponential

p Concave upward if decline is hyperbolic (n>0) or harmonic

2 A plot of q vs Np is

p Linear if decline is exponential

p Concave upward if decline is hyperbolic(n>0) or harmonic

3 A plot of log(q) vs Np is

p Linear if decline is harmonic

p Concave downward if decline is hyperbolic (n<1) or exponential

p Concave upward if decline is hyperbolic with n>1

4 A plot of 1/q vs t is

p Linear if decline is harmonic

p Concave downward if decline is hyperbolic (n<1) or exponential

p Concave upward if decline is hyperbolic with n>1.

Example Exponential decline

Example Rate decline with production

Hyperbolic Decline curve

General Concept of Material Balance.

From: Petroleum Reservoir Engineering

— Amyx, Bass, and Whiting (1960).

a Initial reservoir conditions. b Conditions after producing N p STB of oil,

and G p SCF of gas, and W p STB of water.

Material Balance: Key Issues

̈ Must have accurate production measurements (oil, water, gas).

̈ Estimates of average reservoir pressure (from pressure tests).

̈ Suites of PVT data (oil, gas, water).

̈ Reservoir properties: saturations, formation compressibility, etc

RESERVOIR SIMULATION (RS)

• Reservoir Modelling: primarily the reservoir

engineer’s job

• RS applies the concepts and techniques of math-

ematical modeling to the analysis of the behavior of petroleum reservoir systems

• In a narrower sense å refers only to the

hydro-dinamics of flow within reservoir

• In a larger sense å refer to the total petroleum

system which includes the reservoir, the surface

facilities, and any interrelated significant activity, and economic

• The basic flow model å the partial differential

equations using finite difference methods which

govern the unsteady state flow of all fluid phases in the reservoir medium

RESERVOIR SIMULATOR

Production forecast Optimum production

Reservoir link with surface facility

• Prepare the array input data (maps) of individual flow unit : structure (top & bottom), isopach (net & gross), porosity, permeability, rock compressibility etc.

• Advising to simulation engineer in the designing of

the grid model and layer divisions

• Trace and established in the model grid the

existence of faults, horizontal and vertical barriers

permeability

• During the history matching of production, pressure etc., DG advises to simulation engineer in allowable geological modification such as thickness, structure, rock properties and volumetric reserves

The Role of DG

The Role of DG

in Reservoir Simulation

• PROVED :

– Estimated to reasonable certainty Often based on well logs but normally requires actual production or formation tests.

reservoir or where additional finance is required to produce

• PROBABLE RESERVES

– Less certain than proved but can be assessed to

some degree of certainty May include logging

estimates, improved recovery technique estimates

Decision Making: protocol

application In general, we use this:

investment is based on proved reserves.

economics Internal company decisions usually based on this.

case economics This is the best that could reasonably happen for a venture Companies try to sell ventures based on this.

MM DARISSALAM, YOGYAKARTA JUN ‘08