Some oil wells are perforated on multiple layers to increase production. The individual pressures of the layers are measured to ensure no significant differences which will prevent layer contribution. Single layer PT survey (pressure temperature survey) combined with CBL log (cement bond logging) and electrical logs can answer the question. The production logging tool or PLT also can give layers properties without isolating layers. Sometimes, fluid sampling (surface or downhole sampling) is required to acquire physical properties of fluids produced by the well such as compressibility of liquids.
Individual fluid properties of layers must be compatible. It is important to ensure no precipitation due to fluid mixing. If fluid mixing trigger precipitation, layers can not be produced commingle, one or more layers must be isolated or squeezed.
During well shut-in and waiting for well service, pressure equalization between layers is happening. Fluid from the layer with higher water cut will water saturate and damage other layers. Oil from damage layers needs sometimes to recover back to the condition before saturated by water. Any attempt must be taken to prevent well shut-in. Sometimes, low pressure layer thief fluid from other layers. Shut in well pressure seems low even other layers have high pressure individually.
In Figure 1, the well is completed with multi instruments to detect cross flow between the upper zone and the lower zone. A packer is installed between zones to isolate. Similar completion is not limited to two zones. Flow from each zona is controlled by SSD, choke, valve, or ICD. They can be operated pneumatically, hydraulically, mechanically, or electrically. Cross flow is detected by measuring the rate of change of parameters in both zones. The measuring is made continuously or periodically over time. The parameters such as pressure, temperature, rate, and others if necessary is measured by instruments that held in the carriers. The data are transferred by umbilical to the surface and used to determine whether cross flow has occurred. The operator can close the SSD (or choke, valve, ICD), change the speed of the artificial lift, close a production zone or adjust surface apparatus if cross flow detected.
The P, T, Q (pressure, temperature, rate) instruments can be used to measure skin factor by running a pressure buildup test.
The inventor also provides a processor that can process the data to determine the cross flow condition. The processor uses the nodal method and nodal analysis, neural network or another algorithm to predict cross flow. If cross flow is detected, the processor sends an alarm and gives several options to reduce the effect.