Flowing wellbore pressure

The difference between the flowing wellbore pressure (P, ) and the average reservoir pressure reservoir pressure (P) is the pressure drawdown (AP q). 

The previous sections have considered the flow of fluid to the wellbore. The productivity index (PI) indicates that as the flowing wellbore pressure (Pwf) reduces, so the drawdown increases and the rate of fluid flow to the well increases. Recall... 

With the CBM s unique method of gas storage, the preponderance of the gas is available only to very low coal face pressures. The coal face pressure is set by a combination of flowing wellhead pressure and the hydrostatic head exerted by standing liquid within the wellbore. Effective removal of higher-molecular-weight hydrocarbons and other constituents (liquids removal or deliquification) techniques can reduce or remove the backpressure caused by accumulated liquid. 

An important safety feature on every modern rig is the blowout preventer (BOP). As discussed earlier on, one of the purposes of the drilling mud is to provide a hydrostatic head of fluid to counterbalance the pore pressure of fluids in permeable formations. However, for a variety of reasons (see section 3.6 Drilling Problems ) the well may kick , i.e. formation fluids may enter the wellbore, upsetting the balance of the system, pushing mud out of the hole, and exposing the upper part of the hole and equipment to the higher pressures of the deep subsurface. If left uncontrolled, this can lead to a blowout, a situation where formation fluids flow to the surface in an uncontrolled manner. 

Introduction and Commercial Application Section 8.0 considered the dynamic behaviour in the reservoir, away from the influence of the wells. However, when the fluid flow comes under the influence of the pressure drop near the wellbore, the displacement may be altered by the local pressure distribution, giving rise to coning or cusping. These effects may encourage the production of unwanted fluids (e.g. water or gas instead of oil), and must be understood so that their negative input can be minimised. 

Having reached the wellbore, the fluid must now flow up the tubing to the wellhead, through the choke, flowline, separator facilities and then to the export or storage point each step involves overcoming some pressure drop. 

The purpose of the well completion is to provide a safe conduit for fluid flow from the reservoir to the flowline. The perforations in the casing are typically achieved by running a perforating gun into the well on electrical wireline. The gun is loaded with a charge which, when detonated, fires a high velocity jet through the casing and on into the formation for a distance of around 15-30 cm. In this way communication between the wellbore and the reservoir is established. Wells are commonly perforated after the completion has been installed and pressure tested. 

Oil reservoirs are layers of porous sandstone or carbonate rock, usually sedimentary. Impermeable rock layers, usually shales, and faults trap the oil in the reservoir. The oil exists in microscopic pores in rock. Various gases and water also occupy rock pores and are often in contact with the oil. These pores are intercoimected with a compHcated network of microscopic flow channels. The weight of ovedaying rock layers places these duids under pressure. When a well penetrates the rock formation, this pressure drives the duids into the wellbore. The dow channel size, wettabiUty of dow channel rock surfaces, oil viscosity, and other properties of the cmde oil determine the rate of this primary oil production. 

The rate of flow that would be produced by a well if the only pressure against the face of the producing formation in the wellbore equals atmospheric pressure. 

Filter-cakes are hard to remove and thus can cause considerable formation damage. Cakes with very low permeability can be broken up by reverse flow. No high-pressure spike occurs during the removal of the filter-cake. Typically a high-pressure spike indicates damage to the formation and wellbore surface because damage typically reduces the overall permeability of the formation. Often formation damage results from the incomplete back-production of viscous, fluid loss control pills, but there may be other reasons. 

Cleaning of the wellbore region through the acids and gas fi-om in situ fermentation The gas serves to push oil from dead space and dislodge dehris that plugs the pores the average pore size is increased and, as a result, the capillary pressure near the wellbore is made more favorable for the flow of oil... 

Flow in undisturbed rock normally is radial toward a site of lower pressure (the wellbore). The fracture crack created by high pressure injection usually forms perpendicular to the least principle stress that exists in the rock. The induced fracture intersects and disrupts the radial flow pattern such that flow becomes linear and more direct to the well. This phenomenon has been intensively examined and discussed by authors working in the discipline of rock mechanics as applied to hydrocarbon reservoirs. Hydraulic fractures created in oil and gas wells grow mainly vertically, parallel to the wellbore as depicted in Figure 1 and extend on either side of the perforated wellbore as "wings11 (7-11). 

As an example of how the dump option might be used, consider the problem of predicting whether scale will form in the wellbore as groundwater is produced from a well (Fig. 2.10). The fluid is in equilibrium with the minerals in the formation, so the initial system contains both fluid and minerals. The dump option simulates movement of a packet of fluid from the formation into the wellbore, since the minerals in the formation are no longer available to the packet. As the packet ascends the wellbore, it cools, perhaps exsolves gas as it moves toward lower pressure, and leaves behind any scale produced. The reaction model, then, is a polythermal, sliding-fugacity, and flow-through path combined with the dump option. 

Blowout - A uncontrolled flow of gas, oil or other well fluids from a wellbore at the wellhead or into the formation, caused by the formation pressure exceeding the drilling fluid pressure. It usually occurs during drilling on unknown reservoirs. 

Diverter - The part of the bell nipple at the top of a marine riser, that controls the flow of gas or other fluids that may enter the wellbore under pressure, before the BOP stack has be set in place. It is used when drilling through shallow underground gas zones for diverting gas kicks in deep high pressure zones. 

Foam properties vary with changes in pressure and temperature thus foam qualities increase as they flow away from the wellbore. The property is advantageous when treating an interval with more than one zone having different pressures. In the lower pressure zone, the foam will have a higher quality and potentially more diverting effects. 

Here, rwe is the effective wellbore radius. The skin effect is defined as any factor at or around the wellbore that causes a change in the pressure drop, as compared with the pressure drop that occurs when the stratum is homogeneous and the well fully penetrates the sand (45-48). It is also implied that the inertial effects are attributed to the skin effects because Darcy s law is used to model the flow and hence the skin effects can be different for higher production rates. 

Invasion of Foreign Particles. Invasion of solid particles can cause severe damage around the wellbore. In this case, the solids plug some of the pores and, as a result, the formation permeability diminishes. According to Wojtanowicz et al. (71), the decline of permeability with respect to time can give an indication of the plugging mechanism. At a constant flow rate experiments, the pressure response that occurs due... 

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