OIL RECOVERY MECHANISM

AND

MATERIAL BALANCE EQUATIONS

OIL AND GAS RESERVOIRS

OIL RECOVERY MECHANISMS

INTRODUCTION

EACH RESERVOIR HAS A UNIQUE COMBINATION OF:

- GEOLOGICAL FORM

- ROCK PROPERTIES

- FLUID CHARACTERSTICS AND

- PRIMARY DRIVE MECHANISM

THEREFORE, TWO RESERVOIRS ARE NEVER IDENTICAL.

• OBSERVED THAT EACH DRIVE MECHANISM HAS CERTAIN TYPICAL PERFORMANCE CHARACTERISTICS IN TERMS OF:

- ULTIMATE RECOVERY - PRESURE DEPLETION RATE - GOR - WATER PRODUCTION

• THE RECOVERY OF OIL AND GAS BY ANY NATURAL DRIVE MECHANISM IS CALLED AS PRIMARY RECOVERY, i.e, WITHOUT THE USE OF ANY PROCESS THAT SUPPLIMENT THE NATURAL ENERGY OF THE RESERVOIR.

OBJECTIVES OF THE STUDY

• TO STUDY VARIOUS PRIMARY RECOVERY MECHANISMS AND THEIR EFFECTS ON THE OVER ALL PERFORMANCE OF OIL AND GAS RESERVOIRS.

• TO STUDY THE BASIC PRINCIPLES OF MATREIAL BALANCE EQUATION AND OTHER GOVERNING RELATIONSHIPS THAT CAN BE USED TO PREDICT THE VOLUMETRIC PERFORMANCE OF RESERVOIRS

PRIMARY RECOVERY MECHANISMS

• THE RECOVERY OF OIL AND GAS BY ANY NATURAL DRIVE MECHANISM IS CALLED AS PRIMARY RECOVERY, i.e, WITHOUT THE USE OF ANY PROCESS THAT SUPPLIMENT THE NATURAL ENERGY OF THE RESERVOIR.

• FOR UNDERSTANDING THE RESRVOIR BEHAVIOR AND PREDICTING FUTURE PERFORMANCE, KNOWLEDGE OF DRIVING MECHANISM IS NECESSARY THAT CONTROLS THE BEHAVIOR OF FLUIDS WITHIN RESERVOIRS.

• THERFORE, PERFORMANCE OF THE RESERVOIR IS LARGELY DETERMINED BY THE NATURE OF ENERGY AVAILABLE FOR DRIVING THE OIL TO WELLBORE.

TYPES OF DRIVE MECHANISMS

• THERE ARE BASICALLY SIX DRIVING MECHANISMS THAT PROVIDE THE NATURAL

ENERGY NECESSARY FOR OIL RECOVERY:

- ROCK AND FLUID EXPANSION DRIVE - DEPLETION DRIVE - GAS CAP DRIVE - WATER DRIVE - GRAVITY DRAINAGE DRIVE AND - COMBINATION DRIVE

ROCK AND LIQUID EXPANSION

• IN UNDERSATURATED OIL RESERVOIR, AT PRESSURE ABOVE A BUBBLE – POINT PRESSURE, OIL, CONNATE WATER AND ROCK ARE ONLY PRESENT.

• ROCK AND FLUIDS EXPAND AS RESEVOIR PRESSURE DECLINES.

• ROCK COMPRESSIBILITY IS THE RESULT OF TWO FACTORS:

- EXPANSION OF THE INDIVIDUAL ROCK GRAINS - FORMATION COMPACTION

• THESE FACTORS ARE THE RESULTS OF A DECREASE OF FLUID PRESSURE IN PORE SPACES AND BOTH TEND TO REDUCE THE PORE VOLUME THRU REDUCTION OF POROSITY.

ROCK AND LIQUID EXPANSION

• EXPANSION OF FLUIDS AND REDUCTION IN PORE VOLUME WITH

DECREASING RESERVOIR PRESSURE FORCED THE OIL AND WATER OUT OF POR SPACE TO THE WELL BORE.

• SINCE ROCK AND FLUIDS ARE SLIGHTLY COMPRESSIBLE THERE WOULD BE A SHARP RESERVOIR DECLINE.

• SUCH RESERVOIR SHOW A CONSTANR GOR EQUAL TO GAS SOLUBILITY AT THE BUBBLE POINT PRESSURE.

• THIS DRIVING MECHANISM IS LEAST EFFICIENT

• OIL RECOVERY IN SMALL PERCENTAGE OF THE TOTAL OIL IN PLACE.

DEPLETION DRIVE MECHANISM

• THIS DRIVE IS CALLED BY VARIOUS TERMS:

-SOLUTION GAS DRIVE

- DISSOLVED GAS DRIVE

- INTERNAL GAS DRIVE

• IN THIS TYPE OF RESERVOIR PRINCIPILE SOURCE OF ENERGY IS THE GAS LIBERATED FROM THE OIL AND SUBSEQUENT EXPANSION OF THE SOLUTION GAS AS RESERVOIR PRESSURE IS REDUCED.

• AS THE PRESSURE FALLS BELOW THE BUBBLE POINT PRESSURE GAS BUBBLES ARE LIBERATED AND THEN EXPAND AND FORCE THE OIL OUT OF PORE SPACE.

DEPLETION DRIVE RESERVOIR CHARACTERISTICS

• RESEVOIR PRESSURE: THE RESERVOIR PRESURE DECLINE RAPIDLY AND CONTINUOSLY ATTRIBUTE TO THAT NO EXRANEOUS FLUIDS OR GAS CAP ARE AVAILABLE TO PROVIDE REPLACEMENT OF GAS AND OIL WITHDRAWALS.

• WATER PRODUCTION: ABSENCE OF WATER DRIVE INDICATE NO OR LITTLE WATER PRODUCTION WITH OIL DURING THE PRODUCITION HISTORY.

• GOR: RAPIDLY INCREASE IN GOR IN ALL THE WELLS REGARDLESS OF THEIR STRUCURAL POSTION BELOW THE BUBBLE POINT.

- ONCE THE GAS SATURATION EXCEEDS THE CRITICAL GAS SATURATION FREE GAS BEGINS TO FLOW TO THE WELLBORE.

- IT ALSO BEGIN A VERTICAL MOVEMENT DUE TO GRAVITATIONAL FORCES THERBY FORMING A SECONDAEY GAS CAP.

• ULTIMATE OIL RECOVERY:

• OIL PRODUCTION USUALLY TREATED AS LEAST EFFICIENT RECOVERY METHOD AS IT RESULT IN TO THE FORMATION OF GAS SATURATIOIN THRU OUT THE RESERVOIR.

• ULTIMATE OIL RECOVERY VARY FROM <5% TO ABOUT 30%.

• LOW RECOVERY INDICATES THAT LARGE QUANTITY OF OIL LEFT IN RESERVOIR AND CONSIDERED FOR SECONDARY RECOVERY APPLICATIONS.

PERFORMANCE OF SOLUTION GAS DRIVE RESERVOIR

GAS CAP DRIVE RESERVOIRS

• RESERVOIRS CHARACTERISED BY A SLOW DECLINE IN RESERVOIR PRESSURE.

• SUCH RESERVOIRS IDENTIFIED BY THE PRESENCE OF GAS CAP WITH OR WITHOUT

WATER DRIVE.• TO PRODUCE OIL THE NATURAL ENERGY COMES

FROM TWO SOURCES: - EXPANSION OF GAS CAP

- EXPANSION OF THE SOLUTION GAS AS IT IS LIBERATED

SCEMATIIC DIAGRAM SHOWS GAS CAP RESERVOIR

GAS CAP RESERVOIRS

Gas-cap-drive reservoir

CHARACTERISTICS OF GAS-CAP

RESEVOIRS • THESE ARE :

• RESERVOIR PRESSURE: FALLS SLOWLY AND CONTINOUSLY Pressure maintenance depends upon volume of gas in

gas cap compared to oil volume.

• WATER PRODUCTION: ABSENT OR NEGLIGIBLE• GOR: RISES CONTINOUSLY IN UP–TRUCTURE WELLS• ULTIMATE OIL RECOVERY: HIGHER RECOVERY EFFICIENCY IS DUE TO THAT NO

GAS SATURATION IS BEING FORMED IN RESERVOIR• EXPECTED OIL RECOVERY RANGES FROM 20 TO 40%

CHARACTERISTICS OF GAS-CAP RESERVOIRS

• WELLBORE BEHAVIOR: BECAUSE OF EFFECTS OF GAS CAP EXPANSION ON

PRESSURE MAINTENANCE AND THE EFFECT DECREASED LIQUID COLUMN WEIGHT AS IT IS PRODUCED OUT THE WELL AND THEREFORE FLOW LONGER.

• ULTIMATE OIL RECOVERY VARY DEPENDING ON SIX PARAMETERS

SIZE OF THE ORIGINAL GAS CAP VERTICAL PERMIBILITY OIL VISCOSITY GEGREE OF CONSERVATION OF THE GAS OIL PRODUCTION RATE DIP ANGLE

GAS CAP RESERVOIR

GAS CAP RESERVOIR

EFFECT OF GAS CAP SIZE ON ULTIMATE OIL RECOVERY