Deciphering the downhole pressure differential or overbalance created by relative magnitude of the drilling fluid pressure and formation (or pore) pressure, is paramount for safe and cost-effective drilling. A simplistic approximation of the bottomhole mud pressure would be mud pressure gradient multiplied by the vertical hole depth. For accuracy, additional factors such as cuttings load, frictional effect, and annular velocity can be introduced and applied in two scenarios—one under static conditions, equivalent static density (ESD), and the other under dynamic conditions, equivalent circulating density (ECD). Pore pressure prediction during drilling is performed using logging-while-drilling (LWD) data, drilling parameters, and events analysis (under static conditions). This paper focuses on viewing pressure indicators using an integrated illustration of the occurrence of drilling events under static conditions on a single plot named the drilling event chart and the chart's role in prevention of pressure kick events.
The most important drilling indicators that can act as precursors to an influx or kick event are the magnitude and trend of recurring gas events (connection/pumps-off/background gas). Also, the effect of different types of mud on gas levels is a key factor. The concept of solubility of gas in synthetic-oil-based-mud (SOBM) under high bottomhole mud pressure is very useful, especially to understand that the magnitude of the gas levels can be subdued in SOBM. Knowing the influence of factors such as lithology and rock permeability on the magnitude of gas peaks is equally critical. Hence, honouring the trend of gas peaks/levels rather than noting only the absolute magnitude in isolation is vital. Other than these gas events, indicators such as sudden increase in rate of penetration (ROP), cavings rate/volume, drop in ECD, drop in standpipe pressure (SPP), pit-volume increase, and increase in torque can indicate a decrease in bottomhole overbalance.
Visualization of all these parameters on a single chart has proven useful especially in absence of LWD data for cost-constrained drilling campaigns. This tool, the drilling event chart, entails information such as mud weight, ESD, ECD, gas events (connection/pumps-off gas, background gas), other abnormal drilling events (cavings, high torque, etc.), and pumps-off time. Connection/pumps-off gas events when normalized with respect to pumps-off time establish a profile of normalized gas events that gives a qualitative understanding of the pore-pressure profile. Interpretations based on the observed variation in drilling events under the prevailing mud-weight/ESD/ECD profiles have been useful in decluttering the cause of these events.
The use of this tool for early kick detection has been successful in various basins around the globe. Although the relationship of the drilling events with the bottomhole overbalance is known, their combined usage in this type of single tool provides a quick and novel approach to better understand the cause of these events and accurately evaluate the overbalance in the borehole for safe and efficient drilling.