• Rocks. General Information
• Rock Destruction and Drilling Methods
• Rock Cutting Tools
• Roller Cutter Drill Bits
• Diamond Drilling Tools
• Grinding-Cutting Drill Bits
• Milling Drill Bits
• Auxiliary Drilling Tools
• Drill String
• Steel Drill Pipes
• Light Alloy Drill Pipes
• Drill Collars
• Tool Joints
• Drill Pipe Subs
• Drill String Operation Conditions
• Well Flushing Out and Cementing Hydraulics
• Physical properties of dispersed systems
• Justification of the Density of the Flush Fluid
• Justification of Flush Fluid Flow Rate
• Selection of Hydraulic Downhole Motor
• Selection of Drill Pumps and Operational Conditions
• Well Cementing Hydraulics

Rocks. General Information

Rocks are specific solid bodies. The mathematical models of rocks are very complex.  That is why particular determinations are widely used for their study. Such particular determinations are based on application of simple models, which sufficiently represent a rock in a particular case, as well as on physical modeling and experimental research of physical processes.

Oil and gas fields are confined to sedimentary rocks. This section of the document provides description of mechanical processes of rock destruction while well drilling.

Sedimentary rocks are subdivided into two large groups: crystalline rocks and fragmental rocks. Subgroup of clay rocks is described together with fragmental rocks, though they are of special interest. The share of crystalline rocks in sedimentary rocks is about 25%, 21% falls to fragmental rocks, and clay rocks amount to 54%.

From the point of well drilling, the most important geological characteristics of rocks are mineral composition and heterogeneity. 

At present, about 2000 minerals are known, but only small part of them can be found in rock structures. Such minerals are termed Rock-forming minerals.

Rock that consists of one mineral is termed Monomineral rock (for instance: dolomite, limestone and anhydrate), and rock that consists of several minerals is termed Polymineral rock (for instance: granite, clay and polymictic sandstone).

Rocks are characterized by two basic parameters – structure and texture.

Rock structure means structural features which depend on size, form and nature of surface of crystallites or fragments of fragmental rocks that form a rock.

Rock texture means structural features which depend on relative spatial positions of crystallines or fragments of fragmental rocks.  The textural features include bedding (stratification), fissibility, porosity and fracturing.

Mechanical properties of rock significantly depend on the structural and textural features.

Sedimentary crystalline rocks are mineral aggregate (crystallites) formed due to salt setting from water solutions or due to chemical reactions in the Earth’s crust.  Sedimentary crystalline rocks also include organogenic rocks which are products of living organism activity. Carbonates (limestone, dolomite and chalky clays), sulfates (gypsum and anhydrate), halides (mineral salt) and silica rocks (diatomite and silicon) are most widely occurred.

Sedimentary fragmental rocks include products of mechanical and physicochemical destruction of the earlier existed rocks with consequent transportation by water or wind and settling without dissolving and re-crystallizing.  Rocks formed in place of destruction also relate to this group.

By fragment sizes, the rocks are subdivided into four basic structural subgroups.

Rudaceous rocks (size of fragments is more than 2 mm) are mainly fragments of igneous and metamorphic rocks. Space between fragments is filled with sand, fine-grained and clay rocks. Occurrence of rudaceous rocks in oil and gas deposits is insignificant.

Sand rocks (size of fragments is 0.1-2 mm), to which sands (loose) and sandstones (consolidated) relate, can be, depending on mineral grain composition, silica sand and polymictic sandstones.  Silica sand consists of quartz grains and polymictic sandstone consists of grains of various minerals (mica, hornblende, pyroxene, feldspar and other).

Sands and sandstones differ by grain-size composition: coarse-grained (2-1 mm), large-grained (1-0.5 mm), medium-grained (0.5-0.25 mm), fine-grained (0.25-0.10 mm), homogenous (grains of similar size) and consertal (grains of various sizes).

Fine-grained rocks (size of fragments is from 0.01 up to 0.10 mm) take the intermediate position between sand and clay rocks. Mainly, sediments of continental origin relate to such rocks: sand loam, clay loam and loess). Consolidated rocks, siltstones, widely occur.

Clay rocks (size of particles is less than 0.01 mm) include clay, siltstone and clay shale. Siltstone, usually cemented with chalcedony (SiO₂), has high strength. If siltstone is easily split along plane of fissibility, it is termed clay shale.

Bonding forces in clay rocks are of physicochemical nature, and they are effected by special colloidal state of clay minerals.