Pressure drawdown

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

The relationship between the flowrate (Q) towards the well and the pressure drawdown is approximately linear, and is defined by the productivity index (PI). 

It is common practice to record the bottom hole pressure firstly during a flowing period (pressure drawdown test), and then during a shut-in period (pressure build-up test). During the flowing period, the FBHP is drawn down from the initial pressure, and when the well is subsequently shut in, the bottom hole pressure builds up. 

In the simplest case, for a pressure drawdown survey, the radial inflow equation indicates that the bottom hole flowing pressure is proportional to the logarithm of time. From the straight line plot ot pressure against the log (time), the reservoir permeability can be determined, and subsequently the total skin of the well. For a build-up survey, a similar plot (the so-called Horner plot) may be used to determine the same parameters, whose values act as an independent quality check on those derived from the drawdown survey. 

From downhole pressure drawdown and build-up surveys the reservoir permeability, the well productivity index and completion skin can be measured. Any deviation from previous measurements or from the theoretically calculated values should be investigated to determine whether the cause should be treated. 

In the JAPEX/JNOC/GSC program, small-scale field production tests were conducted by using pressure-drawdown experiments. Data measured in response to depressurization are pressure, temperature, gas flow, and water flow. During our study, these data were not available to us. 

The quantities in equation 20 must be evaluated at the pressure existent at the radius x. This, in general, is not known but, if the well is shut in the average reservoir pressure can be measured. In using equation 20 j3, v, r, fig, and fig are usually evaluated at the average reservoir pressure. This is equivalent to assuming that production occurs at zero pressure differential, that is, with a zero pressure drawdown. 

Procedure. The sequence of flow experiments for each oil started with the core saturated with the live oil at saturation pressure, 4.83 MPa. The pressure at the inlet end was always maintained at 4.83 MPa. A back-pressure regulator (BPR) was used to maintain constant pressure at the outlet end, and this pressure was reduced in steps of approximately 0.34 MPa, starting from the saturation pressure. The term pressure drawdown will be used in the following discussion to denote the pressure difference between the inlet end and the outlet end of the sand pack. The pressure drawdown reaches its maximum value when the pressure at the outlet end becomes equal to the atmospheric pressure. 

At each setting of the outlet pressure, the steady-state pressure profile and the production rate were measured. Therefore, for each oil, 10—12 flow experiments were conducted. End-point average gas saturations (5 ) were estimated after the flow test at the highest pressure drawdown and are presented in Table II. 

The effects of the pressure drawdown on the oil-production rate are shown in Figure 2. Both the experimental data and theoretical calculations based on the Darcy s law are shown. Initially the oil-production rate increased linearly with the increasing pressure drawdown across the porous medium, in agreement with Darcy s law for single-phase flow through porous media. At low drawdown pressures, the dissolved gas remained largely in solution, and therefore, the oil was flowing as a... 

Pressure Distributions in the Sand-Pack. Profiles that show the pressure distribution in each flow experiment are presented in Figures 3 and 4. Each pressure profile in Figures 3 and 4 represents the pressure distribution in the porous medium at a particular pressure drawdown when the steady-state flow was attained. A general trend among these pressure profiles is obvious. The pressure distribution in the porous medium remained linear when the drawdown pressure was below a certain pressure, beyond which the pressure distribution started becoming increas-... 

Figure 3. Experimental pressure pro-files at various pressure drawdowns in the Lindbergh system.

A comparison of the pressure-drop patterns presented in Figure 5 and 6 demonstrates that the drawdown pressure significantly influenced how the pressure in each individual segment was distributed and balanced under different drawdown pressures. For example, at the pressure drawdown of 1 MPa, the segmental pressure drop remained constant, and this pressure drop was accompanied by a steady oil production rate as implied... 

Since equation 39 is linear, superposition of component solutions is permitted. It becomes straightforward to construct solutions for a finite domain reservoir for multiple production and injection wells as long as they are far away from each other for pressure buildup, that is, turning off a production well and for pressure drawdown. 

The reservoir boundaries are normally determined by the constant rate pressure drawdown test. A plane boundary at a distance l from the well would have the same effect on the pressure as the presence of a second well producing at the same rate and at a distance 21 from the first well in a reservoir of infinite size. An analytical solution can be found through superposition for the bottom-hole pressure pw, p(r = rW9 t), in which the effect of the boundary is represented by the effect of the second (image) well (49) ... 

The influence of shear stress and pressure drawdown on solids production has been demonstrated in large-scale laboratory tests (38). Short bursts of solids, from perforation clean-up, and productivity improvement occurred after each increase in effective stress or drawdown. 

Veeken et al. (24) reviewed predictive models for solids production. Their assessment was that modeling of compressive failure is only qualitatively useful. This is because of the sensitivity of the results to the choice of yield envelope and failure criterion. Even so, this approach can be used to develop perforation strategy (density, phasing, and size), to select the stronger zones for perforation, and to provide guidelines for operation of a well (e.g., pressure drawdown, flow rate). 

Proposed mechanisms of solids production from unconsolidated sand reservoirs have been discussed (102,103). Dusseault and Santarelli (104) proposed a mechanism for massive solids production from poorly consolidated sandstones that was based on a general plastic yield of the reservoir brought about by a high pressure drawdown in the yielded region. The vertical stress that the reservoir experiences was also a contributing factor. Subsequently, Geilikman et al. (105-108) developed a model for continuous solids production from unconsolidated heavy oil reservoirs as a yield front propagation. This is different from predictive models previously discussed (45, 46), which dealt with transient and catastrophic production but which did not discuss continuous production explicitly. 

S = 0.32). Furthermore, the pressure decrease becomes more significant closer to the wellbore (i.e. r = 0.2m). Physically, because water is a less viscous and more mobile fluid than oil, less energy (i.e. a lower pressure drawdown) is needed to drive it into the wellbore consequently, the increase of water relative permeability raises the pore pressure whereas that of oil relative permeability lowers it. The synthesis of both effects indicates that pore pressure in a water-dominant fluid system is relatively higher than in an oil-dominant fluid system. 

The form of the subsidence profile suggests diffusion of pore pressure drawdown both north and south from the major inflow point at the tunnel. Most certainly there is also diffusion in the East-West direction within the conductive faults intersected by the tunnel. However, there is little data on the pattern of subsidence in these directions. Nonetheless, it seems reasonable to assume that the penetration of drainage along the major fracture zone that produced the largest inflow was extensive. In the 2-D mcxlels, it was assumed that the fracture is sufficiently conductive that is can be taken as constant potential feature at... 

Pressure Drawdown Tests. This series of experiments were designed to determine the effect of pressure reduction on the stability of certified gas composition between a maximum filling pressure of 300 psig and atmospheric pressure. Two sets of cylinders filled with a 10-component gas blend, similar in composition to our NBS primary standard, will be analyzed before and after each 50-psig pressure reduction at different gas withdrawal rates. The results of these tests will indicate the optimum rate of gas withdrawal as well as the residual cylinder pressure for the GC calibration gas. 

Initialization procedures. Well test field procedures fall into two varieties, namely, pressure drawdown and pressure buildup. Very often, an initially quiescent, pressurized reservoir of uniform pressure (typically equal to the farfield pressure Pr in an aquifer-driven flow) is opened to a lower well pressure P and allowed to produce. The pressure in the neighborhood of the... 

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