The New Hydraulic Fracture Design Method and Good Performance in a Deep and Naturally Fractured Volcanic Gas Reservoir in China

Abstract

Abstract Due to the existence of multi-fracture and the extension in volcanic gas reservoirs, the success ratio of stimulation is very low as well as the stimulating scale. Based on the unique simulation model for fracturing development and growth during the stimulation, some important topics were studied:the effect of the natural micro-fracture and stimulating parameters vs. the formation and extension of the stimulated fractures.The magnitude of the affecting by different characters of the natural micro-fracture in the reservoir vs. the scale and success ratio of treatments. A new method was summarized to determine the operation indicators in real time on site. And this paper also introduced a series of main fracture controlling technique including: plug with rubber plug, plug with silty sand, displacement compensation at far end of the wellbore, etc. Two logging data from two wells in a large-scale stimulating process are presented in this paper, which proves the necessity and validity of the studying for the main fracture controlling technique in volcanic rocks. This method can also be used as a good reference for stimulation in the similar gas reservoirs. I. Distribution and characteristics of deep special-lithology reservoir The reservoir space of volcanic rocks can be classified into 3 categories: original interstice, secondary pore and fracture. It is generally accepted that there used to be massive amount of gas cavities, uninterconnected though in the rocks during the phase when the volcanic rocks took shape. Thanks to the fractures resulted from magmatic condensation and contraction and later stage tectonic movements, the cavities are interconnected thereafter, forming reservoir space in the volcanic rocks. The porosity of volcanic rocks is typically 6.3%--10.8%, and the permeability is 0.55 × 10-3 to 122.0 ×10-3 µm, moreover, the high angle fractures and breaking fractures are well developed. II. Establishment of physical and mathematical models In the process of conventional sandstone fracturing, the filtration rate of the fracturing liquid is virtually constant. But during volcanic rock fracturing, the filtration rate of the fracturing liquid varies markedly. The curve match results of the volcanic rock fracturing demonstrate exponential or gradient filtration rate rise with time. This leads to the conclusion that conventional hydraulic fracturing theory doesn't fit fracturing in the volcanic rocks. Results both from study of volcanic rock fault ductility and Daqing Oilfield's actual core experiments show that the faulted factor of volcanic rocks is slightly higher than that of the average sandstone, and the faulted factor doesn't fluctuate impressively in spite of the presence of microfractures. In order to set up mathematical models, Japanese experts set up equivalent physical model, to make the fractures in different lengths vertical to the axial line of the well bore equivalent to multiple fractures in the same length, and then, by simplifying the mathematical model, ‘equivalent study’ is conducted. To begin with, multiple factors should be defined, and the obvious deviations from actual physical phenomenon which are likely to be caused by ‘equivalence’ will be described by mathematical methods. After that, ‘description correctness’ risk evaluation on equivalent factors will be carried out, and the factors with big risks will be eliminated, meaning that real description instead of ‘equivalent’ description must be done to some items. The factors can only serve the purpose of modification, and can not play a dominant role, that is, with the basic theory unchanged, the factors can only contribute to the study of the influence of variable on the basic form, and the variable is time primarily. When the ‘equivalent’ model is adopted in the study of the initiation and extension of the actual multiple fractures, the crucial ‘equivalent points’ selected include the form changes of the fractures, actual formation- contact filtration area and the influence of multiple fractures' simultaneous extension on the widths of other fractures, meanwhile, the number of the fractures should be taken into account. So, by determining the quantity of the extension of the multiple fractures (volume factor), filtration fractures (filtration factor), and overlap fractures of extension fracture width (opening factor), we study the break and extension situation of the volcanic rocks. Three factors are involved in the mathematical model in the following modes: