Fast prediction method of formation dip angles in horizontal wells based on conventional natural gamma-ray logging

Conventional natural gamma-ray logging (GR) is a standard and commonly used technique in horizontal wells. However, the absence of azimuthal information limits the quantitative methods available for evaluating the formation dip angle, a critical parameter. To this end, a fast prediction method of formation dip angle based on GR is proposed. Firstly, a fast forward method of GR responses in horizontal wells is established using the gamma-ray flux integration formula. Secondly, the GR responses at different angles between the well trajectory and formation boundary (AWF) are simulated and analyzed. Thirdly, based on the logging response patterns, a general equation for calculating AWF is derived. Meanwhile, the group average (GA) and moving average (MA) methods are used to pick the formation boundary interval which corresponds to GR responses. Furthermore, a set of functional relationships among the deviation angle, AWF and formation dip angle are derived in different formation models. After identifying the formation boundary interval, the AWF can be obtained, thereby determining the formation dip angle. In applications, synthetic data tests and two field examples show that the prediction method, only relying on conventional GR, can be used to predict formation dips rapidly and accurately. In addition, the adaptability, advantages, disadvantages and limitations of the method are discussed in detail. The method provides a fast and efficient evaluation solution of formation dip angle in thin interbedded layers and structurally complex areas.