Most of the oil and gas wells are completed with casing. The annulus behind the casing is normally cemented. Once the casing is cemented, it is hermetically insulated from the formation. To produce any fluid from the formation, the casing is perforated using perforation guns. Such perforated completions provide a high degree of control over the pay zone- place where oil is financially fruitful-.because selected intervals can be-perforated and stimulatpeed and/or tested as desired. It is also believed that hydraulic fracturing and sand control are more successful in perforated completions. However, the perforations impose are more successful in perforated complerestrictions to flow from the formation to the wellbore in the form of extra pressure losses. Consequently, if not adequately designed and understood, perforations maysubstantially reduce the flow rates from a well.
Shaped charge perforating is the most common and popular method of perforating. A typical cross section of a shaped charge is shown.
As the shaped charge is detonated, the various stages in the jet development are shown in the next picture below. The velocity of the jet tip is in excess of 30,000 ft/sec, which causes the jet to exert an impact of some 4 million psi on target. Every shaped charge manufacturer provides a specification sheet for charges as to the length of penetration and diameter of the entrance hole in addition to other API required specifications.
PRESSURE LOSS IN PERFORATIONS
The effect of perforations on the productivity of wells can be substancial. Therefore, much work is needed to calculate the pressure loss through perforation tunnels. A very brief review of the background work in the area was presented by Karakas and Tariq (1988).
Most of the calculations on perforation pressure losses are based on single phasegas or liquid flow. It is generally believed that if the reservoir pressure is below the bubblepoint causing two phase flow through the perforations, the pressure loss may be an order of magnitude higher than that for single phase flow. Perez and Kelkar (1988) presented a new method for calculating two phase pressure loss across perforations. we have to consider that two methods of calculating the pressure loss in perforations are presented with appropriate examples. These methods were proposed by McLeod (1983) and Karakas and Tariq (1988).
Mc Leod Method:
Pressure loss in a perforation is claculated using the modified Jones, Blount, and Glaze equations proposed by Mc Leod.
Mc Leod ( 1988) treated an individual perforation tunnel as a miniature well with a compacted zone of reduced permeability around the tunnel. It is believed that compacted zone is created due to the impact of the shaped charge jet on the rock. however, there is no physical means to actually calculate the permeability of the compacted zone. Mc Leod suggested from his experience that the permeability of the compacted zone is:
10 % of the formation permeability, if perforated overbalanced, and
40 % of the formation permeability, if perforated underbalanced.
These numbers may be different in different areas.
The thickness of the crushed zone is assumed to be 0.5 in. The massive reservoir rock surrounding a well perforatin tunnel renders it feasible to assume a model of an infinite reservoir surrounding the well of the perforation tunnel.
Karakas and Tariq Method:
For practical purposes, Mc Leod’s method gives fair estimates of pressure loss through perforations.
However, this model is not very sophisticated to consider the effects of the phasing and spiral distribution of the perforations around the wellbore. Karakas and Tariq (1988) presented a semianalytical solution to the complex problem of three-dimensional (3D) flow into a spiral system of perforations around the wellbore. These solutions are presented for two cases.
- A two-dimensional (2D) flow problem valid for small dimensionless perforation spacing ( large perforation penetration or high-shot density). The vertical component of flow into perforations is neglected.
- A 3 D flow problem around the perforation tunnel, valid in low-shot demsity perforations.
Karakas and Tariq ( 1988) presented the perforation pressure losses in termss of pseudoskins, enabling the modification of the IPR curves (INFLOW PERFORMANCE RATE) to include the effect of perforations on the well performance as follows.
The damaged skin factor, Sdp, is the treatable skin component in perforated completion. The perforation skin, Sp, is a function of the perforation phase angle Θ, the perforation hole radius rp, the perforation shot density n4, and the wellbore radius rw. The following dimensionless parameters are used to correlate the different components of the perforation skin, Sp.
PRESSURE LOSS IN GRAVEL PACK
Gravel packing is done to control sand production from oil and gas wells. Sand production often becomes a production problem causing a reduction in hydrocarbon production, eroding surface and downhole equipment leading to casing collapse.
Although the dynamic of gravel packing is not in the scope of this manual, a typical cross section of a gravel packed well is presented.
The figures show the flow path the reservoir fluid has to follow to produce into the wellbore. Evaluation of well performance in a gravel-packed well thus requires an accounting of the pressure losses caused by the flow through the gravel packs. Jones, Blount, and Glaze equations are adapted with minor modifications to account for the turbulence effects for the calculation of the pressure loss through the gravel packs. These modifies equations for oil and gas cases are presented.
Flow through tubing and flowlines:
Fluid flow through the reservoir and completion are very vital to have the product used. However, the performance evaluation of a well is not complete until the effects of the tubing and flowline are considered with the other effects – pressure loss while transporting, density of the fluid-. The plumbing system includes the tubing and the flowline. Besides, the knowlewdge of Multiphase flow must be compulsory learnt.