Use of neutron generators for monitoring the increase in petroleum recovery from wells due to ultrasonic action

Abstract

The decrease in petroleum output from a well due to degradation of the permeability in the extraction zone of the productive fluid is a frequent negative phenomenon which occurs when a deposit is being developed. This effect is inevitable, since the extraction zone of the productive fluid operates as a filter, retaining and accumulating during well operation all pos sible impurities in the pores of the formation. One of the most effective methods for cleaning the extraction zone is to expose it to a longitudinal ultrasonic pressure wave. In so doing, nonstationary oscillating microflows appear in the fluid-filled pores in the formation. If the acoustic field is strong enough, these microflows can promote the removal of these impurities from the fluid extraction zone. In this case, the petroleum output from the well should increase [1]. In addition, ultrasonic action on the formation and the space around the well is very likely to cause water to flow into the well from behind the main shaft of the well, resulting in the dis placement of the water-petroleum contact and other undesirable and difficult-to-control effects. Thus, there are conditionally three states of the porous medium subjected to acoustic action: 1 ‐ pores are filled with impurities (initial state), 2 ‐ pores are filled with products of hydrocarbon fluid, and 3 ‐ pores are filled with water. These states and the processes resulting in a transition of one state into another must be followed by remote-controlled means through a casing pipe using transparent monitoring methods. Neutron methods are most effective [2]. Examples of such methods are the integral and pulsed neutron‐neutron methods with thermal-neutron detection, integral and pulsed neutron γ-logging with detection of radiative-capture γ-rays, and neutron activation of oxygen. These methods can be implemented using large in-well neutron generators based on sealed accelerator tubes with flux exceeding 10 8 sec ‐1 in a full solid angle [3]. Such generators make monitoring much safer, because a generator in the switched-off state does not create radiation fields and, consequently, serious ecological problems do arise in an accident, which cannot be completely prevented. A block diagram of a neutron monitoring system is presented in Fig. 1. SNM-Type 3 He-filled detectors are used to detect thermal neutrons. The nuclear reaction 3 He(n, p)T occurs in the interior volume of these detectors. γ Rays are detected with a NaI(Tl) or CsI(Na) single crystal scintillation detector with a combined filter for the direct neutron and γ radiation of the in-well generator. The integral neutron‐neutron and neutron γ-logging are based on changing the spatial distribution of thermal neutrons and radiative-capture γ rays depending on the moderating, diffusion, and absorbing properties of the medium being studied. They make it posssible to record the change in the count rate of thermal neutrons or γ rays as a result of a transition from state 1 into state 2 or 3, which are characterized by a substantial concentration of hydrogen atoms in the pores. The hydro