The best well results are achieved through optimum well placement and flow monitoring.

Although the industry has invested heavily in developing ways to improve recovery, the fact is that in most operations more than 60% of the hydrocarbons are left in the ground. A better sense of well performance allows for improved production management.

A new approach

The Production Profile Predictor Method (3PM), a recently patented reservoir and wellbore surveillance technology, allows well performance to be accurately monitored by determining the depletion of each column of fluids in the reservoir draining into the wellbore. The 3PM approach can be used on any well type from vertical to horizontal, including subsea wells, producing in steady-state gas and at least one liquid phase. The well also can have one or more mid-production depths (commingled production well). This process allows the well to be analyzed flowing into a manifold, along with other wells, without either surface or downhole fluids flow rates measuring devices. For well analysis, 3PM requires:

The two measured steady state pressures (PWH1 and PWH3) are represented as Test 1 and Test 3 at the wellhead. GLR represents gas-liquid ratio. Test 2 is not necessarily representative of the well being analyzed. It has the average wellhead pressure and average fluids flow rates of Test 1 and Test 3. (Images courtesy of Production Testing Services)

Two measured steady-state pressures taken either at the wellhead or from a downhole pressure gauge;

Estimates of fluids flow rates at each of the two pressure tests;,

Black oil PVT (pressure, volume, temperature) data; and

Well geometry data. The method determines whether or not the pressures are steady-state. Each of the measured pressure values is at a different total fluids flow rate from the well. The accuracy of the results is independent of the depth of pressure measurements. Fluids flow rate estimates are required to decrease the computing time for a 3PM analysis. These estimates do not affect the accuracy of the computed flow rates at each pressure test.

How it works

In a test that illustrates how 3PM works, two measured steady-state pressures (PWH1 and PWH3) are represented as Test 1 and Test 3 at the wellhead. GLR represents gas-liquid ratio. Test 2 is not necessarily representative of the well being analyzed. It has the average wellhead pressure and average fluids flow rates of Test 1 and Test 3.

From the wellhead +30.5 m (100 ft, arbitrarily chosen) to mid-production depth D1, analysis of the difference of the pressure gradient change between Test 1 and Test 2 and the pressure gradient change between Test 2 and Test 3 shifts from a maximum at the wellhead +30.5 m to a minimum at mid-production depth D1. To identify mid-production depth D1 (where fluids flow rates change) 3PM determines, from the wellhead to the wellhead +30.5 m, the maxi- mum difference of the pressure gradient change between Test 1 and Test 2 and the pressure gradient change between Test 2 and Test 3.

This maximum difference is found using unique accurate liquid flow rates for each test and varying the approximate gas flow rates for each test to obtain the maximum difference between the measured steady state pressures of Test 1 and Test 3. The methodology identifies this maximum difference using any multiphase flow correlation and black oil PVT models.

To calculate unique accurate liquid flow rates and approximate gas flow rates (depending on the multiphase flow correlation and black oil PVT models used) for each test and an accurate mid-production depth, the difference of the pressure change between the tests is accurate and unique for all three tests. This is the case from the wellhead up to mid-production depth D1 with the pressure for Tests 1 and 3 both at depth D13.

Given the accurate measured steady-state pressures at the wellhead for all three tests and the accurate pressure for both Tests 1 and 3 at depth D13, the accurate pressure profiles for all three tests can be determined from the wellhead to mid-production depth D1. Assuming the measured pressures are accurate, if a maximum difference described cannot be obtained, either one or both of the measured pressures are in transient flow. This is evident from transient flow conditions producing each measured pressure with a different mid-production depth.

In general this process of obtaining a maximum difference to determine the results of accurate liquid flow rates and pressure profiles for all three tests is repeated from mid-production depth D1 to D1 +100 ft to mid-production depth D2 through the sequence up to the deepest mid-production depth.

This image illustrates the U-tube effect.

Results

The 3PM well analysis delivers valuable information. It identifies the total accurate liquid and approximate gas flow rates from all columns of fluids draining into the wellbore. This is particularly useful in subsea wells that cannot be individually tested for total fluids flow rates. It also accurately measures the depth interval of each column of fluids draining into the wellbore.

This result is obtained for any such column using an equivalent U-tube effect from basic physics. One side of the U-tube (the source) is a column of fluids flowing in a porous medium from the top of the gas column to the bottom of the water column, both of any radial extent. The other side is the wellbore, also of any ID, with pipe flow. The two sides are connected through the perforations. From the bottom of the water column to the top of the gas column, the difference between the pressure in the column and the pressure in the wellbore decreases due to less driving force needed up the well-bore. For there to be steady-state flow in the wellbore, there has to be an abnormal pressure profile in the well-bore within the column compared to that above and below the column. This is due to the influence of flow of the column in the porous medium, the source. This new phenomenon occurs regardless of the orientation of the U-tube, from vertical to horizontal, and improves current well performance determination.

Accurate mid-production depth is provided within each column described. For each mid-production depth, there are accurate liquid and approximate gas flow rates for all three tests, so water or gas coning can be determined. The approximate Productivity Index (PI) or potential to flow for each fluid type produced also is provided for each mid-production depth. The measured depth intervals of wellbore fluids entry is graphically displayed, as are the measured depths where there are wellbore restrictions (such as wax or paraffin or hydrate buildup) or ID increases.

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