ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 6 potx

Deep connate water show a wide range of Eh and pH depending on their history and how much they’ve mixed with meteoric water.Oilfield brines tends to be more alkaline and more strongly re

CHAPTER 06

THE SUBSURFACE

ENVIRONMENT

1- GROUND WATER AND

TEMPERATURE

1.1 – GROUND WATER

1.1.1 – Origin of ground water

1.1.2 – Chemistry of ground water

1.2 – TEMPERATURE

1.2.1 – Subsurface Temperature

1.2.2 – Regional thermal Variations

1.2.3 – Local thermal Variations

04 Types of GW

Meteoric water

Infiltration of rainwater

Distribution @ shallow depth

Total mineralization: Low

04 Types of GW (cont.)

Juvenile water

Primary of magmatic origin

Brought to near – surface environment dissolved in magma

Usually mixed with either connate or meteoric water

1.1 2 – Chemistry of ground water

Connate water, meteoric water and mixed water can

be differentiated on the basics of their chemistry

Way can be done:

First:

Eh: Oxidation/reduction potential and

pH: Measure of acidity or alkalinity of the water

Fig 01

Deep connate water show a wide range of Eh and pH depending on their history and how much they’ve mixed with meteoric water.

Oilfield brines tends to be more alkaline and more strongly reducing than seawater

The Eh and pH of pore fluids control the precipitation and dissolution of cements such as the carbonates and ion oxides, as well as the alterations of clays minerals

in subsurface rocks  Extremely important to understand the relationships of Eh and pH to diagenesis and the evolution of porosity

Chemistry of ground water (cont.)

Second: Salinity

In general salinity of GW increases with depth

(normal hydrochemical profile)-Fig.02 The rate of

increases varies from basin to basin, even from place

to place within a particular basin

Typical seawater has a salinity of about 35ppthousand (3.5%)

The salinity of GW range from near zero (in newly introduced meteoric to > 600ppthousand (60%) in connate water within evaporate formation

Fig 02

Reversal hydrochemical profile have been observed

due to two possible causes:

1 Meteoric can be trapped beneath an unconformity and preserved as “Paleoaquifer” with relative low salinity as compared connate water above the unconformity

2 Overpressure: In shale sequences, formation water is trapped

In shale, the increases in salinity with depth is less noticeable than in sandstones: Water moves upwards

in compacting sediments, shale acts at semipermeable membranes preventing salt escaping from the sands

Four major sub environment:

1 Zone 1 (surface → 1km) uniform Zone of

circulating meteoric water Salinity fairly uniform;

2 Zone 2 (1 → 3km) gradually increases with depthSaline formation water is ionized;

3 Zone 3 (3km) Chemically reducing

environment, in which hydrocarbons form Salinity uniform with increasing depth; may even decline

if overpressured;

4 Zone 4 incipient metamorphism with

recrystallization of clays to micas

Oder Catelogy Total dissolved Solids

Oder Catelogy Total dissolved Solids

° Regional isosalinity maps are very useful exploration tools.

Area of high salinity possible indicate stagnant regional are uneffected by meteoric flushing

(where oil/gas accumulation may be preserved)

° Meteoric water differs from connate water both salinity and proportions of dissolved irons

° Meteoric water divided to:

+ High proportions of SO2-4 and Na+

+ High CO2-3 and Na+

Connate water divided to:

+ High proportions of CL- and Mg2+

+ High CL- and Ca2+

° In comparison to seawater, connate water high concentration of soluble chlorine and sodium (Tab 03)

(Br more abundant than is seawater)

° SO2-4 in connate water << seawater:

+ Precipitation of CaSO4

+ SO2-4 reduction by bacterial action, producing

H2S gas

Table 03

° Depletion of Mg2+ in connate water due to dolomitize

° Ca2+ in connate water > in seawater:

+ Release of calcium from dolomitize

• Depletion of potassium probably results from the uptake of that element by clay minerals.

• Connate waters also contain traces of dissolved

hydrocarbons which are not common in normal

sea water (Buckley et al., 1958)

– This is significant for two reasons First, it raises the

possibility of regionally mapping dissolved

hydrocarbons as a key to locating new oil and gas

fields Second, it has some bearing on the migration of

both oil and gas

GW research application in O&G Exploitation &

Exploration (By Tran van Xuan)

• Các mẫu nước mỏ dầu đạt yêu cầu được phân tích để xác định:

– Tổng độ khoáng hóa, một số nguyên tố, ion (Cl-, SO42-, HCO3-,

Na + & K + , Mg 2+ , Ca 2+ …)

– Quan hệ giữa các ion

– Xác lập một số quan hệ tỷ lệ, phân loại theo Sulin

– Đánh giá sự thay đổi của độ tổng khoáng hoá theo chiều sâu.

• Ngoài ra một số mẫu nước còn được tiến hành phân tích hàm lượng vi nguyên tố như I, Br, Sr,….

• Tính toán khả năng sa lắng của canxit, thạch cao và sinh khí CO2 tự do

• Đánh giá nguồn gốc, quá trình biến đổi của nước các mỏ.

GW research application in O&G Exploitation &

Exploration (Cont.)

• + Phân loại Xulin: Phân loại của Xulin dựa trên cơ sở

phân chia các loại nước theo các tỉ số nhất định của các ion, đặc trưng cho các điều kiện thành tạo khác nhau

của nước dưới đất nói chung và đặc biệt với nước dưới đất trong các vùng mỏ dầu khí; vì vậy phân loại này

được sử dụng rộng rãi trong địa chất thuỷ văn các mỏ dầu khí.

• Trên cơ sở xem xét các mối quan hệ (Trong đó rNa+, rCl- … được tính bằng %đl/l):

• Xulin chia nước DĐ thành 4 loại:

• 1 Loại nước sunphat natri có nguồn gốc rửa lũa đại lục, được đặc trưng bằng:

• 2 Loại nước bicabonat natri (nước kiềm) có nguồn gốc đại lục, khí quyển được đặc trưng bằng:

• 3 Loại nước clorua magie liên quan với nguồn gốc biển và

được đặc trưng bằng:

• 4 Nước clorua canxi (nước cứng) có nguồn gốc biến chất sâu

(liên quan với các mỏ dầu khí) được đặc trưng bằng:

<1 và

2 4

2 4

GW Classification by Xulin (Russia)

Fig 03

• Lithostatic pressure is due to the weight of the rock

overburden It is transmitted through the subsurface

by grain-to-grain contacts in the rocks

• The magnitude of this lithostatic pressure at a

particular depth depends on the depth, the density of the overlying rocks, and the acceleration due to

gravity

• The lithostatic pressure gradient increases with depth and is approximately 0.6 psi/ft ( 0.136 kg/cm2 * m )

or ( 13.6 kPa/m )

• The fluid pressure, often called "pore pressure" or

"formation pressure", is applied by the fluids within the pore spaces These fluids exert pressure against

the grains

• When the pressure in the pores is caused only by the weight of the column of fluid in the rocks above, it is

called hydrostatic pressure

• For a column of fresh water with a density of 1

gm/cm3, the hydrostatic gradient is 433 psi/ft (0.0979 kg/cm 2 * m) or ( 9.79 kPa/m) The gradient increases with increasing salinity of the water to about 465

psi/ft (0.1052 kg/cm2 * m) or (10.52 kPa/m) for

typical connate water

In the oil industry, fluid pressure is usually calculated as:

p = 0.052 x wt x d where:

– p = hydrostatic pressure ( psi )

– wt = mud height ( lb/gallon )

– d = depth ( ft )

The overburden pressure, which is also called geostatic

pressure, is equal to the sum of the hydrostatic pressure plus the lithostatic pressure This pressure may also be thought of

as the pressure caused by the weight of water plus sediment per unit area The overburden pressure increases with depth

kPa/m )

• Figure 03 summarizing differences between lithostatic and fluid pressure gradients we might normally expect to see