Patent Publication Number: US-6039128-A

Title: Method and system for obtaining core samples during the well-drilling phase by making use of a coring fluid

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and system, for obtaining core samples during the drilling phase of wells, such as hydrocarbon wells and the like, by making use of a coring or encapsulating fluid. 
     2. Description of the Related Art 
     Particular reference will be made below to drilling of hydrocarbon wells, in the oil sector, but it is obvious that the invention can be extended to any field of application in which it might be necessary to obtain core samples during the drilling phase. 
     As is known, during drilling of a well, core samples must be obtained at predetermined depths intervals, that is samples of underground formations must be collected in order to carry out the necessary analyses at the surface to ascertain the type of rock that is being penetrated and identify any traces of hydrocarbon mineralisation, detection of which is the main purpose of well drilling. 
     According to the current methodology, coring operations take place according to the sequence below: 
     drilling is suspended 
     mud is circulated in the well for a long enough period to ensure that formation fluids do not enter the bore hole 
     the rotating drill pipes are lifted to the surface 
     a special piece of equipment called a corer or core barrel is assembled ready 
     the corer is lowered to bottom-hole by means of the pipes 
     coring is carried out 
     the corer is lifted to the surface 
     the cores are collected. 
     The main aim of the invention is to allow collection of &#34;cores&#34; from bottom-of the well, avoiding performance of the work steps listed above and in particular avoiding removal of the rotating pipes from the well. 
     In the present state of the art of drilling, rock debris produced by the chipping action of the drill bit, which will be referred to henceforth by the English term &#34;cuttings&#34;, are lifted from the well bottom to the surface by the circulation of so-called drilling mud, which, being pumped continually, travels downwards inside the constantly rotating drill pipes, then out through nozzles placed on the drill bit, to return upwards along the hollow annular space existing between the drill pipes and the hole bored by the bit. 
     Thus, the well bottom is continually cleaned, cuttings are removed and lifted to the surface, where they are collected for sampling and analysis at predetermined intervals during drilling. 
     With such a method, the cuttings carried to the surface from the well bottom are in direct contact with the mud that carries them upwards and obviously undergo more or less thorough washing which, though not altering their geological nature, nevertheless masks and makes uncertain the identification of fluid in the formation (water, oil, gas), thus eliminating any practical possibility of a qualitative and quantitative interpretation that would have been highly desirable. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome the drawbacks of the prior art and therefore safeguard the nature of the cuttings during lifting to the surface, to allow optimal analysis of the mineralisation fluid contained in the pore spaces and interstices of the cuttings themselves, and conduct a sound paleontological analysis of the fossils belonging to the sampling depth. 
     Yet another aim of the invention is to achieve economic advantages and superior results with respect to the current practice in the field, in view of the considerable time saved by eliminating the current coring procedure. 
     These aims are achieved, with the method and arrangement of the equipment according to the invention, thanks to the characteristics listed in the appended independent claims. 
     Essentially, according to the invention, in order to obtain core samples of underground formations, drilling and mud circulation in the bore are momentarily suspended, while a certain volume of coring matrix fluid, that is to say a fluid with an adhesive effect that serves to encapsulate the cuttings, is introduced into the mud circuit at surface. Normal mud circulation is then resumed, pushing the matrix fluid to the well bottom, after which drilling is resumed, so that the matrix fluid passing through the nozzles in the drill bit hits the cuttings in their virgin state as they are formed and incorporates them in a gelatinous mass, protecting them from direct contact with the mud and thus avoiding the washing effect. The cuttings thus coated by the matrix fluid and pushed upward by the mud circulation reach the surface and are collected and analysed. 
     Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non-limiting embodiment, illustrated in the single FIGURE of the appended drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE illustrates a drilling system provided with means that allow core samples to be obtained according to the invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawing, a drilling rig comprises drill pipe (a) rotated, by means that are not shown, in the direction of the arrow (d). 
     The drill pipes (a) end at the bottom in a drill bit (b), and have nozzles (c) on said bit through which flows the drilling mud (f), which fills the well, and is discharged by overflow. The drilling mud is circulated through the drill pipes (a) by means of a pump (g). 
     The arrows (h) and (l), respectively, indicate the descent of the mud inside the pipes and its return towards the surface, in the hole (e) drilled by the rotating bit. 
     The letter (m) indicates the cuttings travelling upward inside the hole (e), lifted by the mud flow (f). The mud overflowing the hole at surface of hole (e) goes into a vibrating screen apparatus (n), that carries out separation of the cutting from the mud, a sample of cuttings for analysis being indicated by (p). 
     The clean mud returning from the well is collected in a tank (q), and from there it is continually pumped by the pump (g) which recirculates it into the drill pipes (a). 
     In the appended FIGURE indicates the bottom of the well bore. 
     According to the invention, in setting up the existing rig, a pump (t) is connected to the connecting pipe (r) between the pump (g) and the drill pipe (a) by means of a pipe (r&#39;). The pump (t) draws a matrix fluid (u) from a special tank (v), to introduce it, as needed, into the drill pipe (a). 
     The core sample matrix fluid (u) is a viscous, colloidal fluid, that serves to encapsulate the cuttings (m) during lifting to the surface, making it possible to achieve the above mentioned advantages. 
     In fact, when the decision is made to obtain core samples in a matrix during drilling of the well, the operating sequence begins by stopping the mud circulating pump (g) and the drive means for rotation of the drill pipe (a). 
     Using the auxiliary pump (t) a set volume of matrix fluid (u) is introduced into the mud circuit through the pipes (r, r&#39;). 
     The pump (g) is then reactivated to resume the normal mud circulation (h), (l) which has been temporarily suspended. The volume of matrix fluid (u) is thus pushed downward through the drill pipe (a) and, after the time necessary for the volume of fluid (u) to arrive in the vicinity of the bit (b), drilling is resumed maximizing the bit penetration in order to obtain a high concentration of cuttings at the well bottom. 
     As the matrix fluid passes through the nozzles (c) of the bit (b), it hits the cuttings (m) in the virgin condition as they are formed and incorporates them in a gelatinous mass, protecting them from direct contact with the mud and thus avoiding the washing effect. The hydraulic sampling operation is limited in time to a period of a few seconds that will be best indicated by operating experience. 
     The cuttings thus coated with the matrix fluid and pushed upward by the mud circulation reach the surface and are collected there in the usual manner and then analysed more suitably to discover the quality and percentage quantity of original interstitial fluid in the geological formation, belonging to the well-bottom depth investigated. 
     From the foregoing description it will be clear that the coring method with the use of a matrix fluid according to the invention achieves the specified aims. 
     It is obvious, however, that the invention is not limited to what is described above and illustrated in the appended drawing, but numerous changes within the reach of an expert can be made to the details and must nevertheless be considered as coming within the scope of the invention itself, as defined by the claims that follow.