Extraction of natural gas
There are six stages in this procedure:
1. Selection and preparation of the site
The operations manager determines the location and area of the well site. The site can be developed for a single well or for a cluster of wells. Wells can be designed to have a single well bore or multiple well bores that extend away from the main well bore in different directions depending on which areas of the reservoir are being targeted. Before drilling operations begin, the site is levelled and the surface is made ready for heavy vehicles and machinery. Erosion and sedimentation controls are put in place and temporary structures and facilities erected.
2. Well planning, drilling, pipeline installation, cementation, perforation
Drilling operations require an appropriate drilling rig together with professional staff, tools, materials and equipment. The first stage of drilling includes the installation of a large diameter steel pipe called the “conductor casing” and cementing it into the ground to stabilise unconsolidated surface formations, and to seal off shallow water bearing formations. The well is then drilled deeper, either vertically or in a specified direction, and subsequent “surface casing” and “intermediate casing”, are placed one inside the other, cemented in place and pressure tested to provide a positive seal between the well bore (casing string) and the porous geological formations, thereby protecting potentially vulnerable reservoirs form contamination by drilling fluids, or oil and gas. The casings also provide strength to contain the anticipated reservoir pressures while drilling. During drilling, a “blow-out preventer” (BOP) is connected to the casing to provide a surface valve that can be used to control the well and enable drilling to continue safely. The BOP is able to withstand a pressure of up to 700 bars.
When the last casing string has been cemented and pressure tested, the steel casings are connected at the surface with a purpose-built, pressure tested “wellhead”, which can withstand full reservoir pressures and provides the ability to control flow from the well. The casing is made of strong individual steel sections that are screwed together with special threads that create a pressure-tight seal. The well is often cased to its total depth with the “production casing”, which is also cemented into the rock and into the adjacent intermediate casing. This cementing supports the production casing and isolates the well bore from the surrounding rock in order to protect important formations and to prevent fluids and gases from moving upward between the casing and the well wall.
After the production casing is cemented, an arrangement of valves to control the flow at the surface, a “Christmas tree”, is connected to the wellhead. The “production tubing” and its “Bottom Hole Assembly” (BHA) are lowered into the well and set in place in the Christmas tree. In order to connect to the reservoir the production casing is “perforated” at the reservoir depth with special, shaped charges. Other valves and safety devices are installed as required. The BHA seals the internal part of the well just above the perforations and directs the flow through the production tubing to the Christmas tree at the surface. All the components of a well are designed and tested to withstand the maximum anticipated pressure and to last for many years of operation.
3. Hydraulic stimulation
A viscous water gel is pumped into the well under high pressure down to the casing perforations at the depth of the reservoir (approx 3000 m below the surface) to fracture the reservoir sandstone or to expand existing fractures at that depth. This is followed by a mixture of additional viscous water gel with “proppant”. The proppant can be quartz sand or inert ceramic beads of certain grain sizes. The gel and proppant mixture is injected directly into the fractures to expand them and to hold them open after hydraulic pressure is removed. A pressure of between 400 and 450 bars was used for the Pg-10 and Pg-11A wells.
The purpose of hydraulic stimulation is to provide a permeable conduit to improve the inflow to the well bore from the otherwise low permeability reservoir layers. When gas is trapped in tight sandstone layers, it is unable to move into the well bore, and then up the production tubing. Stimulation is a way of improving gas production from low permeability, hard-to-flow sandstone in tight gas reservoirs, as those found in Petišovci. Consequently, it enables an improved and more stable flow of gas into the well. Hydraulic stimulation has long been a global, industry-standard process.
The liquid used in hydraulic stimulation is a gel made from water and guar gum, to which proppant is added, together with a very small volume of flow enhancing additives. The additives used are thoroughly tested for safety and supplied with safety data sheets. These are mainly intended to improve the properties of the liquid used in stimulation. A very small volume of slowly acting enzymes are added to the gelled water to cut the guar gum, polymer-like molecular chains and return the water to its low viscosity state for easy flow-back. In all five fracture stimulations of the Pg-10 and Pg-11A wells, the composition of the fluid used was: 93.8% water, 5.9% KCl (potassium chloride to stabilize swelling clays) and just 0.3% other chemicals.
The casing in the well, the completion design and the stimulation treatment design ensure that the stimulation fluids enter the reservoir rock at the designated depth only, and that they do not contaminate important formations, for example, near surface, fresh water-bearing rocks, a source of drinking water.
4. Completion of drilling and management of waste (return) water
After the fracture has closed onto the proppant (healed), the pressure is released and fracture fluid, and sometimes formation water, flows back to the surface tanks. The fracture fluid and formation water is delivered to a licensed waste company for disposal.
5. Production of natural gas
Prior to the beginning of production, a short flowline and gas treatment infrastructure is built to enable the transportation of gas from the well to the gas processing plant and then to the existing export pipeline.
6. Decommissioning the well
When the production of gas from a well is no longer economically viable, the well is safely decommissioned. The well abandonment procedure includes the installation of a cement plug over the perforations and reservoir section to prevent reservoir fluids from entering the well bore. Several plugs may be required to isolate all potential reservoirs, including fresh water and thermal water, as per regulations. Finally, a cement plug is set at the surface to seal the top of the well and to protect any surface fresh water deposits from contamination. The casing is cut off, usually at 1.5 m below ground level, or as required by regulations. A steel plate is finally welded over the end of the pipe, insulated by mastic and buried. Decommissioning is carried out in accordance with established industry procedures. The surface flowline, Christmas tree and wellhead are removed, the topsoil and vegetation are restored to their original state and all waste is removed from the site and deposited with a licensed waste company. The site can then be safely returned to its former usage.