CHAPTER 06 CHAPTER 06 THE SUBSURFACE THE SUBSURFACE ENVIRONMENT ENVIRONMENT 1 1 - - GROUND WATER AND GROUND WATER AND TEMPERATURE 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 1.1 1.1 - - GROUND WATER GROUND WATER 1.1 .1 – Origin of ground water (GW) 04 types of GW 1. Meteoric water. 2. Connate water. 3. Juvenile water. 4. Mixed water. 04 Types of GW 04 Types of GW Meteoric water Infiltration of rainwater. Distribution @ shallow depth. Total mineralization: Low Tens to be Oxidizing pH: Often acidic due to dissolved humic, carbonic and nitrous acids. Connate water Ancient sea water which was trap in the sediment during burial. Differs from seawater both in concentration of dissolved salt and pH, and Eh . 04 Types of GW (cont.) 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. Mixed water Results from the commingling of meteoric, juvenile and connate waters. Usually between the near – surface meteoric water, juvenile and the deeper, more saline connate water. 1.1 .2 1.1 .2 – – Chemistry of ground water 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 Eh >0: Considered to be oxidizing <0: Considered to reducing pH = 7: [H + ]= [OH - ] Considered to be neutral <7: Acidic > 7: Alkaline 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.) 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 [...]... gradient is 433 psi/ft (0 .0979 kg/cm2 * 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 (1 0.52 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,... 1 và . porosity. Chemistry of ground water (cont .) Chemistry of ground water (cont .) Second: Salinity In general salinity of GW increases with depth (normal hydrochemical profile )- Fig.02. The rate of increases. seawater has a salinity of about 35ppthousand (3 .5 %). The salinity of GW range from near zero (in newly introduced meteoric to > 60 0ppthousand (6 0 %) in connate water within evaporate formation. Fig. 1 (surface → 1km) uniform Zone of circulating meteoric water. Salinity fairly uniform; 2. Zone 2 (1 → 3km) gradually increases with depth Saline formation water is ionized; 3. Zone 3 (3 km)