Filemode not working

static void Main(string[] args)
        {
            using (Package Docpackage = Package.Open("HelloWorld.docx"))
            {
                // Create the mail part
                PackagePart mainPart = Docpackage.CreatePart(new Uri("/document.xml", UriKind.Relative), "application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml");

                // and the relationship
                Docpackage.CreateRelationship(mainPart.Uri, TargetMode.Internal,"http://.../officeDocument/2006/relationships/officeDocument");
                 // create the empty document XML
                using (XmlWriter writer = XmlWriter.Create(mainPart.GetStream(FileMode.CreateNew, FileAccess.ReadWrite)))
                {
                    writer.WriteStartElement("w", "document", "http//.../wordprocessingml/2006/main");
                    writer.WriteEndElement();
                }
                  
            }
        }