TECHNICAL FIELD

The present disclosure relates generally to systems and methods for completing a wellbore, and more specifically (although not necessarily exclusively), to systems and methods for sensing of wellbore leaks during cement curing.

BACKGROUND

During completion of a wellbore, the annular space between the wellbore wall and a casing string (or casing) can be filled with cement. This process is referred to as “cementing” the wellbore. A bottom plug may be inserted into the casing string after which cement may be pumped into the casing string. A top plug may be inserted into the wellbore after a desired amount of cement has been injected. The top plug, the cement, and the bottom plug may be forced downhole by injecting displacement fluid into the casing string. Variations in pressure of the displacement fluid may be used to determine the location of the top plug, the cement, and the bottom plug.

These variations in pressure may be small and may not always be detected or may be incorrectly interpreted. Knowing the position of the top plug, and thereby the cement below it, can prevent damage to the well or other errors in the cementing process. For example, variations in the pressure of the displacement fluid when the bottom plug gets trapped at an undesired location in the casing string may be incorrectly interpreted to mean the bottom plug has reached its destination at a float collar at the bottom of the casing string. Knowing the location of the top plug can increase the integrity of the well.

One technique for determining the location of the top plug is disclosed in U.S. Pat. No. 10,400,544, which is incorporated herein by reference, and will be described in greater detail below. In essence, a sacrificial fiber optic cable is attached to the top plug (or a top plug dart) before it is inserted in the casing string after pumping the desired amount of cement. The top plug is forced downhole by pumping a displacement fluid into the casing string. The top plug may be fitted with a locator device, such as a magnetic pickup coil, that generates an electrical signal each time the locator device passes a casing collar. The electrical signal may drive a light emitter to transmit a light pulse along the fiber optic cable. The light pulse may be detectable by a surface receiver and used to count the number of casing collars that the top plug passes and thereby determine the top plug's location within the wellbore.

After it is determined that the top plug has reached the float collar, the cement displaced from the casing string into the annulus will begin to cure. However, certain problems that may have occurred in the cementing process may be difficult to correct after curing.

For example, loss circulation is the total or partial loss of drilling fluids or cement to high-permeability zones, cavernous formations and natural or induced fractures (i.e., loss or “thief” zones) during the drilling or completion of a well. When loss circulation occurs, the drilling fluids or cement enter the loss zone instead of returning up the annulus around the casing. Additionally, these loss zones may also cause leakage of undesired compounds into the drilling fluids and/or cement, which can cause improper curing of the cement. For example, excess water can cause rapid cooling of the cementing mixture, which in turn can reduce the hardening of the cement and the overall structure of the casing.

During a cementing operation, loss zones and leakages are often detected too late. Conventionally, the only way to detect losses are when the returns don't match the flow in the well bore. As the cementing operation, by design, has transients associated with it, the mismatch between the returns and inputs is expected. Due to this, many times a loss zone is never detected or is only detected when the top of cement (as detected, for example, by a bond log) is not where it is expected to be.

To properly complete a well, cementers need to locate the top of cement within the annulus, as well as determine whether problems occurred in the cementing process. Otherwise, expensive remedial action could be required later.

