Real-Time Monitoring

Various methods for monitoring acidizing can be appropriate - both for monitoring actual stimulation effectiveness - but also might be considered for looking at longterm injection performance. One technique was published by Prouvost, L.P. and Economides, M.J.: "Real-Time Evaluation of Matrix Acidizing Treatments." The basis of these procedures is radial flow theory. The procedures, for stimulation monitoring are:

1. Identify an appropriate reservoir model.
2. Compute the initial skin factor with an injection/displacement/falloff test.
3. Compare real-time and simulated pressures and attributes the reducing differences to diminishing skin.
4. Stimulation ends when the difference between the measured and the simulated pressure vanishes.

McLeod and Coulter, 1969, presented a method in which the transient response to the injected fluid is analyzed while the data are collected. The analysis is conducted in discrete intervals because the pressure transients within each interval are dependent on a constant skin effect. Each stage of injection or shut-in during the treatment is considered as a short well test. The transient reservoir pressure response is analyzed using line source diffusivity solutions. The analysis is only valid if skin does not change in the individual test segment and does not allow for continuous assessment of the evolution of the treatment.

In two publications in 1979, Paccaloni, comprehended evolving skin and presented a technique in which instantaneous pressure and rate data are analyzed and related to the evolving skin effect. Paccaloni selected an "acid bank" of finite length and assumed steady-state flow across it. The reservoir transients can overwhelm the pressure differences due to the changing skin. The skin effect is thereby distorted. For example:

• Uses Darcy's equation for steady-state, single phase, radial horizontal flow:

• The concept of an "acid bank radius", r_b, was used as was an effective wellbore radius, r_w' = r_we^-s, to give:

• The wellhead injection pressure is then derived from the bottomhole pressure and skin is inferred. The concept of damage ratio was used.

• The steady state assumption can provide difficulty. For a radial reservoir, the time to pseudo-steady state is shown below and may often be much longer than the treatment time:

The technique proposed by Prouvost and Economides allows a continuous calculation of the skin during the treatment. At any time, the difference between the simulated pressure response, p_sim(t,s_o), and the measured value, p_meas(t,s_o), is the difference in the actual skin used for the simulation, s_o.

The recommended procedures for the pre-treatment injection test were as follows.

• The reservoir fluid in the tubular string is partly or totally displaced into the formation at a matrix rate.
• The injection is stopped before any foreign fluid hits the formation. The evolution of pressure with time during this period is recorded, typically for one hour.
• Pressure falloff analysis is carried out.
• This will only give average values for layered reservoirs.
• The technique can be generalized by varying the injection rate instead of just shutting in. This may be of value in depleted reservoirs.