Gravel packing
Gravel packs can be performed in either open hole or cased hole completions, in well deviations from 0° to 110° and in zone lengths up to a few thousand feet. Systems are available for virtually any well temperature, pressure, and environment. Gravel packed wells can be produced under high drawdown without concern of sand production. Productivity of the open or cased hole gravel packed completion is determined in part by the condition of the reservoir behind the filter cake, quality of the filter cake, and stability of the wellbore.
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Sand-free production, high productivity, and completion longevity are primary objectives for gravel pack operations. To achieve these objectives, operators must be able to perform gravel pack applications under various well conditions. Several techniques are available for dealing with sand production from wells. These range from simple changes in operating practices to completions such as Sand Consolidation and Gravel Packing. The sand control method selected depends on site specific conditions, operating practices, and economic consideration.
Acidising
The purpose of acidising is to stimulate or effectively increase the flow capacity of wells. The increase in flow capacity is accomplished by the acid’s ability to dissolve rock, certain scale, mud and other soluble material, which may be blocking the flow channels. Acids that are commonly used for stimulation are:
i) Hydrochloric acid (HCl)
ii) Hydrofluoric acid (HF)
iii) Acetic Acid
iv) Formic Acid
v) Other Acid Additives
Of the four acids mentioned above, hydrochloric acid is the most widely used due to its high carbonate dissolving ability and low cost. It reacts with limestone to form water, carbon dioxide and calcium chloride. HCI/HF, also known as mud acid, is used exclusively for sandstone reservoirs with little calcium. A pre-flush of 10% HCl is used to dissolve any calcium which is in the pore throats. Hydrofluoric acid is used on sandstone reservoirs since it reacts with siliceous compounds:
SiO2 + 6HF = H2SiF6 + 2H2O
A mixture of 3% HF and 12% HCl, known as mud acid, is used to dissolve clays and and remove mud cakes created during the drilling process. Acetic and Formic acids are used in stimulations where their slower reaction time and ease of inhibition is required. On the basis of cost, these acids are 3 to 5 times more expensive than HCl.
Acid Fracturing
In acid fracturing, the acid is injected at higher rates and pressures , which fractures the reservoir. The acid then travels along the newly created flow path and etches sides of the fracture as well as the matrix pores along the fracture. This method is useful where deep penetration is required.
Hydraulic Fracturing
Hydraulic fracturing is a technique used to allow oil and natural gas to move more freely from the rock pores where they are trapped to a producing well that can bring them to the surface. The technology was developed in the late 1940s and has been continuously improved and applied since that time. Hydraulic fracturing is used to create small cracks in subsurface geologic formations to
allow oil or gas to move toward a producing well. A fracture acts much like a road, speeding up the journey of oil or gas molecules on their way to the wellbore that will produce them. If only water was being pumped into the well, the fracture would gradually close when the operator stopped pumping, and within minutes the formation would be back to its original non-fractured condition. In a hydraulic fracturing job, the fluid pumped into the well contains a proppant (usually sand) to keep the fracture open.
This proppant collects inside the created fracture, so when the fracture tries to close, it cannot, because the proppant is holding it open. The operator has now “constructed a road” that molecules of gas far out in the coal can use to travel to the well. Some of these gas molecules might not have been able to make it to the well otherwise. Even though this new fracture is full of proppant, it is still much more permeable and easier to travel through than the coal itself.
The extent of the fracture is controlled by the characteristics of the geologic formation, its depth, the fluid type, and pumping pressure. The fracture will grow if the operator continues to pump fluid at higher rates, or if the operator pumps a more viscous fluid into the formation (e.g., molasses = high viscosity, water = low viscosity). Whether the fracture grows higher or longer is determined by the surrounding rock properties. When the fracture reaches the shale above (or below) the geologic formation being fractured, it will stop; shale does not fracture easily. In nature, fluids that are under pressure (such as fracturing fluids) will follow the path of least resistance. A hydraulically created fracture will always take the path of least resistance, which means staying within the formation that fractures easiest.
